Optical recording medium and recording/reading method therefor

ABSTRACT

In a single-sided incident type optical recording medium having a plurality of dye containing recording layers, sufficient reflectance and excellent recording characteristics necessary to record or read information in or from a dye containing recording layer positioning farther from a side from which a light beam comes in can be obtained. The optical recording medium has a first substrate ( 21 ) having a guide groove, a first dye containing recording layer ( 22 ), a semitransparent reflective layer ( 23 ), an intermediate layer ( 24 ), a second dye containing recording layer ( 25 ), a reflective layer ( 26 ) and a second substrate ( 27 ) having a guide groove. Information is recorded or read in or from the first dye containing recording layer ( 22 ) and the second dye containing recording layer ( 25 ) by irradiating the light beam from the first substrate&#39;s side. The depth of the guide groove on the second substrate is within a range from 1/100×λ to ⅙×λ where λ represents the recording/reading wavelength.

TECHNICAL FIELD

The present invention relates to an optical recording medium having aplurality of recording layers in which information is recorded or readby irradiating a light beam from one side thereof such as a DVD-R or thelike, a recording/reading method for the optical recording medium, and amanufacturing method of the optical recording medium.

BACKGROUND ART

Various types of optical recording media such as CD-R, CD-RW, MO and sofroth are widely recognized and spread as external storages forinformation processing apparatuses such as computers because they canstore a large volume of information and can be randomly accessed easily.With an increase in quantity of handled information, there is a demandto increase the recording density.

Among various optical recording media, optical disks having a recordinglayer containing an organic dye (also referred to as a dye containingrecording layer) such as CD-R, DVD-R, DVD+R and the like areparticularly widely used because they are relatively inexpensive andhave compatibility with read-only optical disks.

Media such as CD-R representative of optical disks having the dyecontaining recording layer, for example, are in a laminated structurewhich has a dye containing recording layer and a reflective layer inorder on a transparent disk substrate along with a protective layer forcovering the dye containing layer and the reflective layer. Recordingand reading are performed with a laser beam through the substrate.

DVD-R (single-sided, single-layer DVD-R), which is representative aswell, has a laminated structure in which a dye containing recordinglayer, a reflective layer and a protective layer covering them areformed in this order on a first transparent disk substrate, and aso-called dummy disk, which has a second disk substrate (which may betransparent or opaque) and, if necessary, a reflective layer formed onthe second disk substrate, provided on the protective layer through ornot through an adhesive layer.

Recording or reading are performed with a laser beam from one side ofthe disk through the first transparent disk substrate. The dummy diskmay be of only a transparent or opaque disk substrate, or may beprovided with a layer other than the reflective layer.

Meanwhile, DVD+R has almost the same structure as DVD-R, description ofwhich will be hereinafter represented by DVD-R.

In order to largely increase the recording capacity of the opticalrecording medium, two single-sided DVD-Rs as above are bonded togetherto form a medium having two recording layers, which is known as adouble-sided DVD-R (double-sided, dual-layered DVD-R). Recording andreading are performed by irradiating a laser beam onto each of therecording layers from the both sides (that is, the laser beam isirradiated from one side of the medium to perform recording and readingon a recording layer closer to this side, while the laser beam isirradiated from the other side of the medium to perform recording andreading on the other recording layer closer to the other side).

With respect to optical recoding media having a plurality of recordinglayers, there is, in these years, a demand for a single-sided incidenttype optical recording medium (for example, single-sided incidentdual-layered DVD-R) on which recording and reading can be performed on aplurality of recording layers by irradiating a laser beam from one sideso as to avoid an increase in size and complexity of therecording/reading apparatus, enable continuous reading from the pluralrecording layers, and improve the facility.

To meet the above demand, there has been proposed a single-sidedincident type DVD-R of the dual layer type (single-sided, dual-layeredDVD-R) having two recording layers, for example, as a single-sidedincident type optical recording medium having the structure below (referto Japanese Unexamined Patent Publication No. HEI 11-066622, forexample).

For example, a single-sided incident type DVD-R of the dual layer typeof the bonded type is formed by laminating, on a firstlight-transmissible substrate, a first recording layer made from anorganic dye on which information can be optically recorded byirradiating a laser beam for recording, a first reflective layer made ofa semi-light-transmissible reflective film that can pass through a partof the laser beam for reading, an intermediate layer that can passthrough the laser beam for recording and the laser beam for reading, asecond recording layer made from an organic dye on which information canbe optically recoded by irradiating the laser beam for recording, asecond reflective layer reflecting the laser beam for reading, and asecond substrate in this order.

DISCLOSURE OF INVENTION

Generally, a guide groove (concave portion) is formed, spirally orconcentrically, on the substrate of the optical recording medium such asa CD, DVD or the like to guide a recording light beam or a reading lightbeam.

Generally, the depth of the guide groove is approximately 150 nm, forexample, in an optical recording medium having a dye containingrecording layer (hereinafter referred to as a recording layer) such as aCD-R, DVD-R or the like.

When a material for forming a recording layer is coated on the substratein order to make an optical recording medium having the recording layersuch as a CD, DVD or the like, the film thickness of the recording layeris large at the concave portion because the recording layer is suchformed as to fill the concave portion on the substrate. It is generallysaid that when the recording track is formed at a portion (thick filmportion; concave portion) at which the film thickness is large,excellent recording/reading characteristics (for example, reflectance,maximum signal amplitude, polarity, etc.) are obtained.

The maximum signal amplitude is a value obtained by standardizing thesignal amplitude of the longest mark/longest space (14T mark/14T spacein a medium of the DVD system) in terms of reflectance.

For this, the recording track is formed at the thick film portion in allthe commercially available optical recording media.

Meanwhile, the guide groove (concave portion) formed on the substrate isa convex portion when looked from the side from which the light beam isirradiated at the time of recording or reading. Namely, the concaveportion on the substrate is a convex portion of the dye recording layer.

A single-sided incident type optical recording medium (for example, asingle-sided incident type DVD-R of the dual layer type or the like)having a plurality of dye containing recording layers is underdevelopment.

For example, the single-sided incident type optical recording mediumhaving two dye containing recording layers has a first dye containingrecording layer closer to a side from which the light beam is irradiated(light irradiated side, one side), and a second dye containing recordinglayer farther from the same. In such the single-sided incident typeoptical recording medium, recording or reading of information in or fromthe second dye containing recording layer is performed by irradiatingthe light beam through the first dye containing recording layer.

In such the single-sided incident type optical recording medium, whenthe guide groove formed on the substrate positioning on the oppositeside to the side from which the light beam comes in has a depth ofapproximately 150 nm like general optical recording media, there ispossibility that reflectance necessary for recording or reading ofinformation in or from the second dye containing recording layer cannotbe obtained.

When the recording track is formed on the convex portion (thick filmportion) of the first dye containing recording layer like commerciallyavailable optical recording media, excellent recording/readingcharacteristics can be obtained. However, since the conditions of thesecond dye containing recording medium differ from those of the firstdye containing recording layer, there is possibility that morepreferable recording track is different.

In the light of the above problems, an object of the present inventionis to provide an optical recording medium, a recording/reading methodfor the optical recording medium and a manufacturing method for theoptical recording medium, the optical recording medium having aplurality of dye containing recording layers, in which information isrecorded or read by irradiating a light beam from one side thereof,wherein sufficient reflectance and more excellent recording/readingcharacteristics can be obtained when information is recorded in or readfrom a dye containing recording layer positioning farther from the sidefrom which the light beam comes in, or when information is recorded inor read from a dye containing recording layer by irradiating the lightbeam from the opposite side to the substrate.

Therefore, the present invention provides an optical recording mediumcomprising at least a first substrate having a guide groove, a first dyecontaining recording layer, a semitransparent reflective layer, a seconddye containing recording layer, a reflective layer and a secondsubstrate having a guide groove, in which information is recorded in orread from the first dye containing recording layer and the second dyecontaining recording layer by irradiating a light beam from the firstsubstrate's side, wherein a depth of the guide groove of the secondsubstrate is within a range from 1/100×λ to ⅙×λ where λ represents arecording/reading wavelength.

The present invention further provides an optical recording mediumcomprising at least a first substrate having a guide groove, a first dyecontaining recording layer, a semitransparent reflective layer, a seconddye containing recording layer, a reflective layer and a secondsubstrate having a guide groove, in which information is recorded in orread from the first dye containing recording layer and the second dyecontaining recording layer by irradiating a light beam from the firstsubstrate's side, wherein a depth of the guide groove of the secondsubstrate is shallower than a depth of the guide groove of the firstsubstrate.

The present invention still further provides an optical recording mediumcomprising a first information recording body formed by laminating atleast a first dye containing recording layer containing a first dye anda semitransparent reflective layer in this order on a first substratehaving a guide groove and a second information recording body formed bylaminating at least a reflective layer and a second dye containingrecording layer containing a second dye in this order on a secondsubstrate having a guide groove, the optical recording medium beingformed by bonding together the first information recording body and thesecond information recording body through an optically transparentadhesive layer, with opposite sides to the substrates of the firstinformation recording body and the second information recording bodyfacing to each other, in which information is recorded or read byirradiating a light beam from the first substrate's side, wherein adepth of the guide groove of the second substrate is within a range from1/100×λ to ⅙×λ where λ represents a recording/reading wavelength.

The present invention still further provides an optical recording mediumcomprising a plurality of dye containing recording layers, in whichinformation is recorded or read by irradiating a light beam from oneside thereof, wherein a depth of a guide groove used to record or readinformation in or from a dye containing recording layer positioningfarther from the side, from which the laser beam is irradiated, iswithin a range from 1/100×λ to ⅙×λ where λ represents arecording/reading wavelength.

The present invention still further provides an optical recording mediumcomprising at least a dye containing recording layer, a reflective layerand a substrate having a guide groove, in which information is recordedin or read from the dye containing recording layer by irradiating alight beam from an opposite side to the substrate, wherein a depth ofthe guide groove of the substrate is within a range from 1/100×λ to ⅙×λwhere λ represents a recording/reading wavelength.

The present invention still further provides a recording/reading methodfor an optical recording medium comprising a first dye containingrecording layer and a second dye containing recording layer, in whichinformation is recorded in or read from the first dye containingrecording layer and the second dye containing recording layer, which hasa thick film portion and a thin film portion, by irradiating a lightbeam from one side thereof, the recording/reading method comprising thesteps of irradiating the light beam to the thin film portion of thesecond dye containing recording layer through the first dye containingrecording layer to record or read information in or from the second dyecontaining recording layer.

It is preferable that the thick film portion and the thin film portionof the second dye containing recording layer are formed correspondinglyto a concave portion and a convex portion, respectively, on a substrateformed on an opposite side to the side, from which the light beam isirradiated.

It is preferable that the first dye containing recording layer has athick film portion and a thin film portion, and the light beam isirradiated to a thick film portion of the first dye containing recordinglayer to record or read information in or from the first dye containingrecording layer.

It is preferable that the thick film portion and the thin film portionof the first dye containing recording layer are formed correspondinglyto a concave portion and a convex portion, respectively, on a substrateformed on the side, from which the light beam is irradiated.

The present invention still further provides an optical recording mediumcomprising a first information recording body formed by laminating atleast a first dye containing recording layer containing a first dye anda semitransparent reflective layer in this order on a first substratehaving a guide groove and a second information recording body formed bylaminating at least a reflective layer and a second dye containingrecording layer containing a second dye in this order on a secondsubstrate having a guide groove, the optical recording medium beingformed by bonding together the first information recording body and thesecond information recording body through an optically transparentadhesive layer, with opposite sides to the substrates of the firstinformation recording body and the second information recording bodyfacing to each other, in which information is recorded or read byirradiating a light beam from the first substrate's side, wherein thesecond dye containing recording layer has a thick film portion and athin film portion, and the light beam is irradiated to the thin filmportion of the second dye containing recording layer to record or readinformation in or from the thin film portion.

The present invention still further provides an optical recording mediumcomprising a plurality of dye containing recording layers, in whichinformation is recorded or read by irradiating a light beam from oneside thereof, wherein a dye containing recording layer positioningfarther from the side, from which the light beam is irradiated, has athick film portion and a thin film portion, and the light beam isirradiated to the thin film portion thereof to record or readinformation in or from the thin film portion.

The present invention still further provides an optical recording mediummanufacturing method for manufacturing the above optical recordingmedium comprising the steps of forming a guide groove on the secondsubstrate or the substrate with a negative stamper.

According to the optical recording medium, the recording/reading methodfor the optical recording medium and the manufacturing method for theoptical recording medium of this invention, it is possible to attainsufficient reflectance and more excellent recording/readingcharacteristics in an optical recording medium having a plurality of dyecontaining recording layers, in which information is recorded or read byirradiating a light beam from one side thereof, when information isrecorded in or read from a dye containing recording layer positioningfarther from the side, from which the light beam comes in. Wheninformation is recorded in or read from a dye containing recording layerby irradiating the light beam from the opposite side to the substrate,it is possible to attain sufficient reflectance and more excellentrecording/reading characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the whole structure of an opticalrecording medium according to an embodiment of this invention; and

FIG. 2 is a schematic diagram showing the whole structure of anotheroptical recording medium according to the embodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, description will be made of an example of an opticalrecording medium (write-once optical recording medium), arecording/reading method for the optical recording medium, and amanufacturing method for the optical recording medium according to anembodiment of this invention with reference to FIGS. 1 and 2.

(1) Structure of Optical Recording Medium

An optical recording medium according to this invention is asingle-sided incident type optical recording medium having a pluralityof recording layers, in or from which information can be recorded orread by irradiating a laser beam from one side thereof.

The optical recording medium according to this invention will beexplained by way of a single-sided incident type DVD-R of the dual layertype having two recording layers (single-sided, dual-layered DVD-R,single-sided dual-layered DVD-Recordable disk) as a single-sidedincident type optical recording medium of the bonded type (single-sidedincident type DVD-R).

FIG. 1 is a schematic sectional view showing an optical recording medium(optical disk) according to this embodiment.

The optical recording medium according to this invention has, as shownin FIG. 1, a first recording layer (first dye containing recordinglayer) 22 containing a dye, a semitransparent reflective layer(hereinafter referred to as a semitransparent reflective layer) 23, atransparent adhesive layer (intermediate layer) 24, a buffer layer 28, asecond recording layer containing a dye (second dye containing recordinglayer) 25, a reflective layer 26, a disk-shaped second substrate 27 inthis order on a disk-shaped transparent (light-transmissible) firstsubstrate (first light-transmissible substrate) 21. Optical beams areirradiated from the side of the first substrate 21 to perform recordingand reading.

The optical recording medium according to this invention has a firstinformation recording body formed by laminating at least the first dyecontaining recording layer 23 containing a first dye and thesemitransparent reflective layer 13 in this order on the first substrate21 having a guide groove, and a second information recording body formedby laminating at least the reflective layer 26 and the second dyecontaining recording layer 25 containing a second dye in this order onthe second substrate 27 having a guide groove. The first informationrecording body and the second information recording body are bondedtogether through an optically transparent adhesive layer, with theopposite surfaces to the substrates of the first information recordingbody and the second information recording body facing to each other.

In the optical recording medium according to this invention,“transparent (light-transmissible)” signifies “transparent(light-transmissible) to optical beams used for recording and readinginformation in and from the optical recording medium.” Transparent(light-transmissible) layers include a layer which absorbs more or lessthe optical beams used for recording and reading. For example, when thelayer has a transmittance of not less than 50 percent (preferably notless than 60 percent) to the wavelength of an optical beam used forrecording or reading, the layer is considered to be light-transmissible(transparent).

Next, each of the layers will be described.

(a) With Respect to First Substrate 21

It is desirable that the first substrate 21 has excellent opticalcharacteristics, that is, the first substrate 21 is transparent, hassmall birefringence, and so forth. The reflectance (the reflectance tothe wavelength of the recording beam or reading beam) of the firstsubstrate 21 is generally not less than 1.40, preferably not less than1.45, generally not greater than 1.70, and preferably not greater than1.65. It is also desirable that the first substrate 21 has excellentmolding properties, that is, the first substrate 21 can be readilyformed in injection molding. When the first substrate 21 has smallhygroscopicity, such property is desirable because the warping can bedecreased.

Further, it is desirable that the first substrate 21 has shape stabilityso that the optical recording medium has some degree of rigidity. Whenthe second substrate 27 has sufficient shape stability, the firstsubstrate 21 is not always required to have large shape stability.

As such material, it is possible to use resins such as acrylic resins,methacrylic resins, polycarbonate resin, polyolefin resins(particularly, amorphous polyolefin), polyester resins, polystyreneresin, epoxy resin, and so forth, and glass. Alternatively, the firstsubstrate 21 may be comprised of a plurality of layers. For example, itis possible to provide a resin layer made from a radiation-setting resinsuch as a photo-setting resin or the like on the substrate made fromglass, resin, or the like.

Meanwhile, polycarbonate is preferable from the viewpoint of opticalproperties, high productivity such a molding properties and the like,cost, low hygroscopicity, shape stability, etc. From the viewpoint ofchemical resistance, low hygroscopicity and the like, amorphouspolyolefin is preferable. From the viewpoint of high-speedresponsibility and the like, a glass substrate is preferable.

The first substrate 21 is preferably thin. It is preferable that thefirst substrate 21 has a thickness of not greater than 2 mm, morepreferably not greater than 1 mm. The smaller the distance between theobjective lens and the recording layer and the thinner the substrate,the smaller is coma aberration, which is advantageous to increase therecording density. To obtain sufficient optical properties,hygroscopicity, molding properties and shape stability, some degree ofthickness is required. It is thus preferable that the thickness of thefirst substrate 21 is generally not less than 10 μm, more preferably notless than 30 μm.

In order to well perform recording and reading on both of the firstrecording layer 22 and the second recording layer 25 in this opticalrecording medium of this invention, it is desirable to suitably adjustthe distance between the objective lends and the both recording layers.For example, it is preferable to set the focus of the objective lends atan almost intermediate point between the both recording layers becauseaccesses to the both layers become easy.

More concretely, in a DVD-ROM or DVD-R system, the distance between theobjective lens and the recording layer is adjusted to be most suitablewhen the thickness of the substrate is 0.6 mm.

When this layer structure is compatible with DVD-ROM, it is mostpreferable that the first substrate 21 has a thickness obtained bysubtracting a half of the film thickness of the transparent adhesivelayer 24 as being the intermediate layer from 0.6 mm. If so, theapproximately intermediate point between the both layers isapproximately 0.6 mm, thus the focusing servo control can be readilyperformed on the both recording layers.

When another layer such as a buffer layer, a protective layer or thelike exists between the second recording layer 25 and thesemitransparent reflective layer 23, it is most preferable that thefirst substrate 21 has a thickness obtained by subtracting a half of asum of the thicknesses of that layer and the transparent adhesive layer24 from 0.6 mm.

A groove (guide groove) 31 used to guide a recording/reading light(recording/reading beam; for example, a laser beam) for recording orreading information is formed, spirally or concentrically, on the firstsubstrate 21. When the groove 31 is formed on the substrate 21 as this,a concave portion and a convex portion are formed on the surface of thesubstrate 21. The concave portion (groove) of the irregularities iscalled a groove, whereas the convex portion is called a land. The grooveor land is used as a recording track to record or read information in orfrom the first recording layer 22. Incidentally, the groove 31 on thefirst substrate 21 is the convex portion with respect to the directionin which the beam is irradiated.

In the case of a so-called DVD-R disk on which recording and reading areperformed by condensing a laser beam having a wavelength of 650 nm withan objective lens having a numerical aperture of 0.6 to 0.65, forexample, the first recording layer 22 is generally formed in coating onthe first substrate 21 so as to have a larger film thickness at thegroove (concave portion) on the first substrate 21, which is suitablefor recording or reading. It is thus preferable that the groove is usedas the recording track.

The depth (groove depth; the height of the convex portion of the firstdye containing recording layer) of the groove 31 formed on the firstsubstrate 21 is preferably not less than 1/10×λ where λ represents therecording/reading wavelength because reflectance can be sufficientlysecured. The depth of the groove 31 is more preferably not less than⅛×λ, still more preferably not less than ⅙×λ. When the wavelength of therecording/reading beam (recording/reading wavelength) is =650 nm, forexample, the depth of the groove 31 on the first substrate 21 ispreferably not less than 65 nm, more preferably not less than 81 nm,still more preferably not less than 108 nm.

It is preferable that the depth of the groove 31 on the first substrate21 is preferably not greater than 2/4×λ because the transferability ofthe shape of the groove is improved. More preferably, the depth of thegroove 31 is not greater than ⅖×λ, and still more preferably, notgreater than 2/6×λ. When the recording/reading wavelength is λ=650 nm,for example, the depth of the groove 31 on the first substrate 21 ispreferably not greater than 325 nm, more preferably not greater than 260nm, still more preferably not greater than 217 nm.

It is preferable that the width (groove width, G width; width of theconvex portion of the first dye containing recording layer; half width)of the groove 31 on the first substrate 21 is not less than 1/10×T whereT represents the track pitch because sufficient reflectance can besecured. More preferably, the width of the groove 31 is not less than2/10×T, still more preferably, not less than 3/10×T. When the trackpitch is 740 nm, for example, the width of the groove 31 on the firstsubstrate 21 is preferably not less than 74 nm, more preferably not lessthan 148 nm, still more preferably not less than 222 nm.

It is preferable that the width of the groove 31 on the first substrate21 is not greater than 9/10×T because excellent transferability of theshape of the groove can be obtained. The width of the groove 31 is morepreferably not greater than 8/10×T, still more preferably not greaterthan 7/10×T. When the track pitch is 740 nm, for example, the width ofthe groove 31 on the first substrate 21 is preferably not greater than666 nm, more preferably not greater than 592 nm, still more preferablynot greater than 518 nm.

In the case of the groove recording, the groove 31 on the firstsubstrate 21 is made slightly snake in the radial direction at apredetermined amplitude and a predetermined frequency to form wobble. Inthe land between the grooves 31 on the first substrate 21, isolated pits(address pits) are formed in a certain rule (this called Land Pre-Pit;LPP). The address information may be beforehand recorded with the LandPre-Pits. Other concave/convex pits (pre-pits) may be formed asrequired.

From the viewpoint of cost, it is preferable to manufacture thesubstrate having such a concave portion and a convex portion ininjection molding with a stamper having a concave portion and a convexportion. When a resin layer made from a radiation-setting resin such asa photo-setting resin or the like is formed on the substrate made fromglass or the like, irregularities such as a recording track and the likemay be formed on the resin layer.

(b) With Respect to First Recording Layer 22

Generally, the sensitivity of the first recording layer 22 is almostequivalent to that of the recording layer used in a single-sidedrecording medium (for example, CD-R, DVD-R, DVD+R) or the like.

In order to realize excellent recording/reading characteristics, it ispreferable that the first recording layer 22 contains a low-exothermaticdye which is lowly exothermatic and has high refractive index.

The refractive index (refractive index to the wavelength of therecording beam or reading beam) of the dye used in the first recordinglayer 22 is generally not less than 1.00, preferably not less than 1.50,and generally not greater than 3.00.

The extinction coefficient (extinction coefficient to the wavelength ofthe recording beam or reading beam) of the dye used in the firstrecording layer 22 is generally not greater than 0.50, preferably notgreater than 0.30. When the extinction coefficient is excessively large,absorption by the dye containing recording layer becomes excessivelylarge, which causes a decrease in reflectance. However, absorption tosome extent is preferable for the sake of recording. Thus, theextinction coefficient is generally not less than 0.001 although thereis particularly no lower limitation.

Further, a combination of the first recording layer 22 and thesemitransparent reflective layer 23 is preferably within appropriateranges of the reflection, transmission and absorption of the light,whereby the recording sensitivity is improved and the thermalinterference during recording is decreased.

As such organic dye material, there are macrocyclic azaannulene dyes(phtalocyanine dyes, naphtalocyanine dyes, porphyrin dyes, etc.),pyrromethene dyes, polymethine dyes, (cyanine dyes, merocyanine dyes,squalirium dyes, etc.) anthoraquinone dyes, azulenium dyes, metalcomplex azo dyes, metal complex indoaniline dyes, etc.

Among the above various organic dyes, metal complex azo type dyes arepreferable because they have excellent recording sensitivity, durabilityand light resistance. Particularly, a compound represented by thefollowing general formula (I) or (II) is preferable:

(where rings A¹ and A² are nitrogen-containing aromatic heterocycles,each of which can independently have a substituent; rings B¹ and B² arearomatic rings, each of which can independently have a substituent; andX is an alkyl group having carbon number 1 to 6 substituted with atleast two fluorine atoms).

An organic dye used in the recording layer of the optical recordingmedium according to this invention preferably is a dye compound whichhas the maximum absorption wavelength λ max within a range from thevisible rays to the near infrared rays of approximately 350 to 900 nm,and is suited to recording with a laser of blue to near microwave. Morepreferable is a dye suited to recording with a near infrared laser at awavelength from about 770 to 830 nm (typically at 780 nm, 830 nm, etc.)used generally for CD-R, a red laser at a wavelength of about 620 to 690nm (typically at 635 nm, 650 nm, 680 nm, etc.) used for DVD-R, or aso-called blue laser at a wavelength of about 340 to 530 nm (typicallyat 410 nm or 515 nm, etc.).

It is possible to use one kind of dye, or mix two or more the same ordifferent kinds of dyes and use them. Further, it is possible to usetogether dyes suited for recording with a recording beam at a pluralityof wavelengths to realize an optical recording medium coping withrecording with a laser beam in a plurality of wavelength bands.

The first recording layer 22 may contain a transition metal chelatecompound (for example, acetylacetonato chelate, bisphenyldithiol,salicylaldehyde oxime, bisdithio-α-diketone or the like) as a singletoxygen quencher in order to stabilize the recording layer or improve thelight resistance, or a recording sensitivity improving agent such as ametal system compound or the like in order to improve the recordingsensitivity. Here, the metal system compound is that a metal such as atransition metal or the like in the form of atom, ion, cluster or thelike is contained in a compound. As such metal system compound, thereare, for example, organometallic compounds such as ethylenediaminecomplexes, azomethine complexes, phenylhydroxyamine complexes,phenanthroline complexes, dihydroxyazobenzene complexes, dioximecomplexes, nitrosoaminophenol complexes, phyridyltriazine complexes,acetylacetonato complexes, metallocene complexes, porphyrin complexes,and the like. There is no limitation with respect to the metal atom, buta transition metal is preferable.

Further, a binder, a leveling agent, an antiforming agent and the likemay be together used to make the first recording layer 22 of the opticalrecording medium according to this invention as required. As apreferable binder, there are poly(vinyl alcohol), poly(vinylpyrrolidone), nitrocellulose, cellulose acetate, ketone resins, acrylicresins, polystyrene resins, urethane resins, poly(vinyl butyral),polycarbonate, polyolefin, etc.

The film thickness of the first recording layer 22 is not specificallylimited because the suited film thickness differs according to therecording method or the like. However, in order to obtain sufficientmodulation amplitude, the film thickness is preferably not less than 5nm, more preferably not less than 10 nm, and specifically preferably notless than 20 nm, in general. However, the recording layer is requirednot to be excessively thick in order to appropriately pass through thelight in the optical recording medium of this invention. Accordingly,the film thickness of the recording layer is generally not greater than3 μm, preferably not greater than 1 μm, and more preferably not greaterthan 200 nm. The film thickness of the first recording layer 22 differsfrom the groove to the land. In the optical recording medium of thisinvention, the film thickness of the first recording layer 22 is at thegroove on the substrate.

As the method of deposition of the first recording layer 22, there canbe applied a thin film deposition generally performed such as vacuumevaporation, sputtering method, doctor blade method, cast method, spincoating, dipping method or the like. From the standpoint of productivityand cost, spin coating is preferable. Vacuum evaporation is morepreferable than coating method because it can yield a recording layerhaving uniform thickness.

When the deposition is performed in spin coating, the rotation speed ispreferably 10 to 15000 rmp. After the spin coating, a process ofannealing or applying solvent vapor or the like may be performed.

As a coating solvent used when the first recording layer 22 is formed ina coating method such as doctor blade method, cast method, spin coating,dipping method or the like, the type of solvent is not limited, thus anysolvent can be used so long as it does not attack the substrate. Forexample, there are ketone alcohol type solvents such as diacetonalcohol, 3-hydroxy-3-methyl-2-butanone and the like, cellosolve typesolvents such as methyl cellosolve, ethyl cellosolve and the like, chainhydrocarbon type solvents such as n-hexane, n-octane and the like, ringhydrocarbon type solvents such as cyclohexane, methylcyclohexane,ethylcyclohexane, dimethylcyclohexane, n-butylcyclohexane,tert-butylcyclohexane, cyclooctane and the like, perfluoroalkylalcoholtype solvents such as tetrafluoropropanol, octafluoropentanol,hexafluorobutanol and the like, hydroxy carboxylic acid ester typesolvents such as methyl lactate, ethyl lactate, methyl2-hydroxyisobutyric acid and the like, etc.

In the case of vacuum evaporation, recording layer compounds such as anorganic dye and various additives and the like as required are put in acrucible disposed inside a vacuum chamber, for example, the inside ofthe vacuum chamber is evacuated to about 10⁻² to 10⁻⁵ Pa by anappropriate vacuum pump, after that, the crucible is heated to vaporizethe recording layer components, and the recording layer components aredeposited on the substrate placed opposite to the crucible, whereby thefirst recording layer 22 is formed.

(c) With Respect to Semitransparent Reflective Layer 23

The semitransparent reflective layer 23 is a reflective layer havingsome degree of light transmittance. Namely, the semitransparentreflective layer 23 is a reflective layer which has small absorption(absorption of recording/reading beam), a light transmittance of notless than 40 percent, and appropriate light reflectance (of not lessthan 30 percent, in general). For example, by providing a thin metalfilm having high reflectance, it is possible to give appropriatetransmittance. It is desirable that the semitransparent reflective layer23 has some degree of corrosion resistance. Further, it is desirablethat the semitransparent reflective layer 23 has shutting-off propertiesso that the first recording layer 22 is not affected by leaking of theupper layer (here the transparent adhesive layer 24) of thesemitransparent reflective layer 23.

To secure high transmittance, the thickness of the semitransparentreflective layer 23 is preferably not greater than 50 nm, in general.The thickness of the semitransparent reflective layer 23 is morepreferably not greater than 30 nm, and still more preferably not greaterthan 25 nm. However, the semitransparent reflective layer 23 is requiredto be thick to some degree in order to avoid an effect of the upperlayer of the semitransparent reflective layer 23 on the first recordinglayer 22. Thus, the thickness of the semitransparent reflective layer 23is generally not less than 3 nm, and more preferably not less than 5 nm.

As the material of the semitransparent reflective layer 23, it ispossible to use, solely or in the form of alloy, metals and semimetalssuch as Au, Al, Ag, Cu, Ti, Cr, Ni, Pt, Ta, Pd, Mg, Se, Hf, V, Nb, Ru,W, Mn, Re, Fe, Co, Rh, Ir, Zn, Cd, Ga, In, Si, Ge, Te, Pb, Po, Sn, Biand rare earth metals, which have appropriately high reflectance at thewavelength of the reading beam. Among them, Au, Al and Ag have highreflectance, thus are suitable as the material of the semitransparentreflective layer 23. The semitransparent reflective layer 23 may containother component than the above as being the main component.

A material containing Ag as the main component is particularlypreferable because of its low cost and high reflectance. Here, the maincomponent signifies a material contained not less than 50 percent.

Since the semitransparent reflective layer 23 has thin film thickness,large crystal grains of the film cause reading noise. Thus, it ispreferable to use a material having small crystal grains. Since puresilver tends to have large crystal grains, it is preferable to use Ag asin the form of alloy.

Particularly, it is preferable to contain Ag as the main component, and0.1 to 15 atomic percent of at least one element selected from the groupconsisting of Ti, Zn, Cu, Pd, Au and rare earth metals. When two or moreof Ti, Zn, Cu, Pd, Au and rare earth metals are contained, each of thesemay be 0.5 to 15 atomic percent. However, the sum of these is preferably0.1 to 15 atomic percent.

A particularly preferable alloy composition is one that contains Ag asthe main component, 0.1 to 1.5 atomic percent of at least one elementselected from the group consisting of Ti, Zn, Cu, Pd and Au, and 0.1 to15 atomic percent of at least one rare earth metal. Among the rare earthmetals, neodymium is particularly preferable. In more concrete, AgPdCu,AgCuAu, AgCuAuNd, AgCuNd, etc. are preferable.

As the semitransparent reflective layer 23, a layer made from only Au ispreferable because it has small crystal grains and corrosion resistance,but it is more expensive than an Ag alloy.

Alternatively, it is possible to use a layer made from Si as thesemitransparent reflective layer 23.

It is possible to stack, one on the other, a thin film having lowreflectance and a thin film having high reflectance both made frommaterials other than metals to form multi-layers, and use them as thereflective layer.

As a method for forming the semitransparent reflective layer 23, therecan be applied, for example, sputtering, ion plating, chemicalevaporation, vacuum evaporation, etc. It is possible to provide aninorganic or organic intermediate layer and an adhesive layer betweenthe first substrate 21 and the semitransparent reflective layer 23 inorder to improve the reflectance, the recording characteristics and theadhesive properties. For example, it is possible that an intermediatelayer (or an adhesive layer), the first recording layer 22, and anintermediate layer (or an adhesive layer) and the semitransparentreflective layer 23 are stacked in this order on the first substrate 21to provide the intermediate layer (or the adhesive layer) between thefirst substrate 21 and the first recording layer 22, and to provide theintermediate layer (or the adhesive layer) between the first recordinglayer 22 and the semitransparent reflective layer 23.

(d) With Respect to Transparent Adhesive Layer 24

The adhesive layer 24 is required to be transparent. High adhesion andsmall shrinkage of the adhesive layer 24 at the time that the layer ishardened and adhered brings stability of the shape of the medium, whichis preferable.

The refractive index (refractive index to the wavelength of therecording beam or reading beam) of the transparent adhesive layer 24 isgenerally not less than 1.40, preferably not less than 1.45, generallynot greater than 1.70, preferably not greater than 1.65.

It is desirable that the transparent adhesive layer 24 is made from amaterial that does not damage the second recording layer 25. Thetransparent adhesive layer 24 is easily compatible with the secondrecording layer 25 because the transparent adhesive layer 24 isgenerally made from a resin. For this, it is desirable to provide abuffer layer 28 to be described later between the transparent adhesivelayer 24 and the second recording layer 25 in order to prevent thetransparent adhesive layer 24 from dissolving the second recording layer25 and from giving damage thereto.

Further, it is desirable that the transparent adhesive layer 24 is madefrom a material that does not damage the semitransparent reflectivelayer 23. It is possible to provide a known inorganic or organic bufferlayer between the both layers in order to avoid the damage.

In the optical recording medium of this invention, it is preferable toaccurately control the film thickness of the transparent adhesive layer24. The film thickness of the transparent adhesive layer 24 ispreferably not less than 5 μm, in general. It is necessary to provide acertain degree of distance between the two recording layers in order toperform the focusing servo control separately on the two recordinglayers. The film thickness of the transparent adhesive layer 24 isrequired to be generally not less than 5 μm, and preferably not lessthan 10 μm although it depends on the focusing servo mechanism.

Generally, the distance between the two recoding layers can be smalleras the objective lends has a larger numerical aperture. However, whenthe transparent adhesive layer 24 is excessively thick, it takes longtime to adjust the focusing servo to the two recording layers and theobjective lends has to be moved for a long distance, which is thusundesirable. Further, an excessively thick layer requires a long time toharden, which leads to a decrease in productively. Accordingly, the filmthickness of the transparent adhesive layer 24 is preferably not greaterthan 100 μm.

As the material of the transparent adhesive layer 24, available arethermoplastic resins, thermosetting resins, electron beam settingresins, ultraviolet ray-curable resins (including retarded delayedcurable type), etc., for example.

The transparent adhesive layer 24 can be formed by dissolving athermoplastic resin, thermosetting resin or the like in an appropriatesolvent to prepare a coating liquid, applying the liquid, and drying(heating) the liquid. In the case of a ultraviolet-curable resin, thetransparent adhesive layer 24 can be formed by dissolving the resin asit is or dissolving the resin in an appropriate solvent to prepare acoating liquid, coating the coating liquid, and radiating ultravioletrays to harden the resin. There are various types of ultravioletray-curable resins. However, any one of them can be used so long as itis transparent. One of these materials can be used or some of them canbe mixed together to be used. Not only single layer but also multiplelayers are applicable.

As the coating method, a coating method such as spin coating, castmethod or the like is applicable, like the recording layer. Among them,spin coating is preferable. A resin having high viscosity can be coatedin screen printing or the like. Use of a ultraviolet ray-curable resinthat liquidizes at the temperature of 20 to 40° C. is preferable becauseno solvent is necessary to coat the resin. It is preferable to preparethe resin so that the resin has a viscosity of 20 to 1000 mPa·s.

Incidentally, it is possible to use a pressure sensitive double-sidedtape, and put the tape between the laminated structures and press it toform the adhesive layer.

As the ultraviolet ray-curable adhesives, there are radical typeultraviolet ray-curable adhesives and cation type ultravioletray-curable adhesives, both of which are usable.

As the radical type ultraviolet-curable adhesives, all the knowncompositions are available. A composition containing an ultravioletray-curable compound and a photopolymerization initiator as essentialingredients is used. As the ultraviolet ray-curable compound,monofunctional (meta)acrylate or multifunctional (meta)acrylate isavailable as a polymeric monomer ingredient. These can be used solely,or two or more kinds of them can be used together. In this invention,acrylate and metaacrylate will be together referred to as(meta)acrylate.

For example, the followings are the polymeric monomers that can be usedfor this optical recording medium. As monofunctional (meta)acrylate,there is, for example, (meta)acrylate or the like having, as thesubstituent, a group of methyl, ethyl, propyl, butyl, amyl,2-ethylhexyl, octyl, nonyl, dodecyl, hexadecyl, octadecyl, cyclohexyl,benzyl, methoxyethyl, butoxyethyl, phenoxyethyl, nonylphenoxyethyl,tetrahydrofurfuryl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl,3-chloro-2-hydroxypropyl, dimethylaminoethyl, diethylaminoethyl,nonylphenoxyethyltetrahydrofurfuryl, caprolactone denaturatedtetrahydrofurfuryl, isobornyl, dicyclopentanyl, dicyclopentenyl,dicyclopentenyloxyethyl, or the like.

As the multifunctional (meta)acrylates, there are di(meta)acrylates of1,3-butylenegycol, 1,4-butanediol, 1,5-pentanediol,3-methyl-1,5-pentanediol, 1,6-hexanediol, neopentyl glycol,1,8-octanediol, 1,9-nonanediol, tricyrodecandimethanol, ethylene glycol,polyethylene glycol, propylene glycol, dipropylene glycol, tripropyleneglycol, polypropylene glycol and the like, di(meta)acrylate oftris(2-hydroxyethyl)isocyanurate, di (meta) acrylate of diole obtainedby adding 4 or more moles of ethylene oxide or propylene oxide to 1 moleof neopentyl glycol, di(meta)acrylate of diole obtained by adding 2moles of ethylene oxide or propylene oxide to 1 mole of bisphenol A, dior tri (meta) acrylate of triol obtained by adding 3 or more moles ofethylene oxide or propylene oxide to 1 mole of trimethylolpropane,di(meta)acrylate of diol obtained by adding 4 or more moles of ethyleneoxide or propylene oxide to 1 mole of bisphenol A,trimethylolpropanetri(meta)acrylate, pentaerythritoltri(meta)acrylate,poly(meta)acrylate of dipentaerythritol, ethylene oxide denaturatedphospholic acid (meta)acrylate, ethylene oxide denaturated alkylatedphospholic acid (meta)acrylate, etc.

One that can be used together with polymetic monomer is polyester(meta)acrylate, polyether (meta)acrylate, epoxy (meta)acrylate, urethane(meta)acrylate or the like, as polymeric oligomer.

As a photopolimerization initiator used for the optical recording mediumof this invention, any one of the known initiators that can harden aused ultraviolet ray-curable compound represented by polymeric oligomerand/or polymeric monomer can be used. As the optical polymerizationinitiator, the molecular fission type or the hydrogen abstraction typeis suitable.

As such photopolymerization initiator, suitably used are bensoinisobutyl ether, 2,4-diethylthioxanthone, 2-isopropylthioxanthone,benzyl, 2,4,6-trimethylbenzoyldiphenylphosphineoxide,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butane-1-one,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpenthylphosphi noxide, etc. Asthe molecular fission type other than these,1-hydroxycyclohexylphenylketone, benzomethylether, benzyldimethylketal,2-hydroxy-2-methyl-1-phenylpropane-1-one,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropane-1-one,2-methyl-1-(4-methylthiophenyl)-2-morphorinopropane-1-one, and the likecan be together used. Further, benzophenone, 4-phenylbenzophenon,isophthalphenone, 4-benzoyl-4′-methyl-diphenylsulfide or the like, whichare photopolimerization initiator of the molecular abstraction type, canbe together used.

As the sensitizer to the photopolymerization initiator, amine that doesnot cause the addition reaction with the above polymeric component, suchas trimethylamine, methyldimethanolamine, triethanolamine,p-diethylaminoacetophenone, p-dimethylaminoethylbenzoate,p-dimethylaminoisoamylbenzoate, N,N-dimethylbenzylamine,4,4′-bis(diethylamino)benzophenone or the like can be together used. Itis preferable to select and use one of the above photopolymerizationinitiators and sensitizers which has excellent solubility to theultraviolet ray-curable compound and does not hinder the ultraviolet raytransmissivity.

As the cation type ultraviolet ray-curable adhesive, all the knowncompositions can be used. Epoxy resins containing a photopolimerizationinitiator of the cation polymerization type correspond to this. As photoinitiators of the cation polymerization type, there are sulfonium salts,iodonium salts, diazonium salts, etc.

As examples of iodonium salts, there are diphenyliodoniumhexafluorophosphate, diphenyliodonium hexafluoroantimonate,diphenyliodonium tetrafluoroborate, diphenyliodoniumtetrakis(pentafluorophenyl) borate, bis(dodecylphenyl)iodoniumhexafluorophosphate, bis(dodecylphenyl)iodonium hexafluoroantimonate,bis(dodecylphenyl)iodonium tetrafluoroborate, bis(dodecylphenyl)iodoniumtetrakis(pentafluorophenyl)borate,4-methylphenyl-4-(1-methylethyl)phenyliodonium hexafluorophosphate,4-methylphenyl-4-(1-methylethyl)phenyliodonium hexafluoroantimonate,4-methylphenyl-4-(1-methylethyl)phenyliodonium tetrafluoroborate,4-methylphenyl-4-(1-methylethyl)phenyliodoniumtetrakis(penthafluorophenyl)borate, etc.

As the epoxy resin, any one of bisphenol A-epichlorohydrin type,alicylic epoxy, long-chain aliphatic type, brominated epoxy resin,glycidyl ester type, glycidyl ether type, heterocyclic system, etc. isavailable.

As the epoxy resin, it is preferable to use one that has small contentsof liberated free chlorine and chlorine ions in order to avoid the resinfrom damaging the reflective layer. The quantity of chlorine ispreferably not greater than 1 wt %, and more preferably not greater than0.5 wt %.

A rate of the cation polymerization type photo-initiator to 100 parts byweight of the cation type ultraviolet ray-curable resin is generally 0.1to 20 parts by weight, and preferably 0.2 to 5 parts by weight. In orderto use more effectively the wavelengths in the near infrared ray regionor the visible radiation region in the wavelength band of theultraviolet ray source, it is possible to use together a known opticalsensitizer. As such optical sensitizer, there are anthracene,phenotiazine, benzylmethylketal, benzophenone, acetophenone, etc.

In order to improve various properties of the ultraviolet ray-curableadhesive, it is possible to add, as other additives, a thermalpolymerization hinibitor, an antioxidant represented by hindered phenol,hindered amine, phosphite, etc., a plasticizer, a silane coupling agentrepresented by epoxysilane, mercaptosilane, (meta)acrylsilane, etc., asrequired. Among them, one that has excellent solubility to theultraviolet ray-curable compound and does not hinder the ultraviolet raytransmissiveness is selected and used.

(e) With Respect to Second Recording Layer 25

The second recording layer 25 generally has higher sensitivity than arecording layer used for a single-sided recording medium (for example,CD-R, DVD-R, DVD+R) and the like. In this optical recording medium,since the power of an incident optical beam is decreased due to presenceof the first recording layer 22, the semitransparent reflective layer 23and the like, recording is performed with an approximate half of thepower. Accordingly, the second recording layer 25 is required to havespecifically high sensitivity.

For the purpose of realization of excellent recording/readingcharacteristics, it is desirable that the dye develops a little heat andhas large refractive index.

The refractive index (refractive index to the wavelength of therecording beam or reading beam) of the dye used in the second recordinglayer 25 is generally not less than 1.00, preferably not less than 1.50,and generally not greater than 3.00.

The extinction coefficient (extinction coefficient to the wavelength ofthe recording beam or reading beam) of the dye used in the secondrecording layer 25 is generally not greater than 0.50, preferably notgreater than 0.30. When the extinction coefficient is excessively large,absorption by the dye recording layer becomes excessively large, whichcauses a decrease in reflectance. Absorption to some extent ispreferable for recording, thus the extinction coefficient is generallynot less than 0.001 although there is no lower limitation.

Further, it is desirable that a combination of the second recordinglayer 25 and the reflective layer 26 provides appropriate ranges ofreflection and absorption of the light beam. Whereby, the recordingsensitivity can be increased and the thermal interference at the time ofrecording can be diminished.

The materials and deposition method of the second recording layer 25 arealmost the same as the first recording layer 22, thus only thedifferences between them will be hereinafter described.

The film thickness of the second recording layer 25 is not specificallylimited because the suitable film thickness differs according to therecording method, etc. In order to obtain sufficient modulationamplitude, the film thickness of the second recording layer 25 ispreferably not less than 10 nm in general, more preferably not less than30 nm, and particularly preferably not less than 50 nm. However, thefilm is required not to be excessively thick in order to obtainappropriate reflectance, thus the film thickness is generally notgreater than 3 μm, preferably not greater than 1 μm, and more preferablynot larger than 200 nm. Here, the film thickness of the second recordinglayer 25 is generally a film thickness at the thick film portion.

The materials used for the first recording layer 22 and the secondrecording layer 25 may be the same or may differ from each other.

(f) With Respect to Reflective Layer 26

The reflective layer 26 is required to have high reflectance. It isdesirable that the reflective layer 26 is highly durable.

In order to secure high reflectance, the thickness of the reflectivelayer 26 is preferably not less than 20 nm, in general, more preferablynot less than 30 nm, and still more preferably not less than 50 nm. Inorder to shorten the tact time of the production and decrease the cost,it is preferable that the reflective layer 26 is thin to some degree.Accordingly, the film thickness is generally not greater than 400 nm,and more preferably not greater than 300 nm.

As the material of the reflective layer 26, it is possible to use,solely or in a form of alloy, metals having sufficiently highreflectance at a wavelength of the reading light such as Au, Al, Ag, Cu,Ti, Cr, Ni, Pt, Ta and Pd, for example. Among them, Au, Al and Ag aresuitable for the material of the reflective layer 26 because they havehigh reflectance. Other than these as the main compositions, thereflective layer 26 may contain the followings as other components. Asexamples of the other components, there are metals such as Mg, Se, Hf,V, Nb, Ru, W, Mn, Re, Fe, Co, Rh, Ir, Cu, Zn, Cd, Ga, In, Si, Ge, Te,Pb, Po, Sn, Bi and rare earth metals, and semimetals.

A film containing Ag as the main component is particularly preferablebecause the cost thereof is low, it provides high reflectance and abeautiful white ground color when a print accepting layer to bedescribed later is further provided. Here, “main component” signifies acomponent whose rate of content is not less than 50 percent.

In order to secure high durability (high corrosion resistivity) of thereflective layer 26, it is preferable to use Ag in the form of alloyrather than as pure silver.

Among the alloys, an alloy that contains Ag as the main component andcontains 0.1 to 15 atomic percent of at least one element selected fromthe group consisting of Ti, Zn, Cu, Pd, Au and rare earth metals ispreferable. When the alloy contains two or more of Ti, Zn, Cu, Pd, Auand rare earth metals, each of them may be contained 0.1 to 15 atomicpercent. However, it is preferable that the sum of these is 0.1 to 15atomic percent.

A particularly preferable composition of the alloy is that Ag iscontained as the main component, 0.1 to 15 atomic percent of at leastone element selected from the group consisting of Ti, Zn, Cu, Pd and Auis contained, and 0.1 to 15 atomic percent of at least one rear earthelement is contained. Among rare earth elements, neodymium isparticularly preferable. More concretely, AgPdCu, AgCuAu, AgCuAuNd,AgCuNd or the like is preferable.

As the reflective layer 26, a layer made from only Au is preferablebecause of its high durability (high corrosion resistance), but is moreexpensive than a layer made from an Ag alloy.

It is possible to stack a thin film having low reflective index and athin film having high reflective index, both of which are made frommaterials other than metals, one on the other to form a multilayer, anduse it as the reflective layer 26.

As a method for forming the reflective layer 26, there are, for example,spattering, ion plating, chemical vapor deposition, vacuum evaporation,etc. It is possible to provide a known inorganic or organic intermediatelayer or an adhesive layer on the upper surface and the lower surface ofthe reflective layer 26 in order to improve the reflectance, recordingcharacteristics, adhesive properties and so forth.

(g) With Respect to Second Substrate 27

It is preferable that the second substrate 27 has shape stability sothat the optical recording medium has some degree of rigidity. Namely,it is preferable that the second substrate 27 has high mechanicalstability and large rigidity.

As such material, there can be used resins such as acrylic resins,methacrylic resins, polycarbonate resin, polyolefin resins (particularlyamorphous polyolefin), polyester resins, polystyrene resin, epoxy resinand so forth, and glass.

The second substrate 27 may be comprised of a plurality of layers. Forexample, a resin layer made of a radiation-setting resin such as aphoto-setting resin or the like may be formed on a substrate made ofglass, a resin or the like to be used as the second substrate.

When the first substrate 21 does not have sufficient shape stability asabove, the second substrate 27 is particularly required to have largeshape stability. In this viewpoint, it is preferable that the secondsubstrate 27 has low moisture absorption.

The second substrate 27 is not required to be transparent. When thesecond substrate 27 is transparent, the refractive index (refractiveindex to the wavelength of the recording beam or reading beam) of thesecond substrate 27 is generally not less than 1.40, preferably not lessthan 1.45, and generally not greater than 1.70, preferably not greaterthan 1.65.

As such material, the same material as that used for the first substrate21 can be used. Other than this, there can be used an Al alloy substratemade from an Al—Mg alloy or the like containing Al as the maincomponent, an Mg alloy substrate made from a Mg—Zn alloy or the likecontaining Mg as the main component, a substrate made from any one ofsilicon, titanium and ceramics, or a substrate made by combining them.

In the viewpoint of high productivity such as molding property and thelike, cost, low moisture absorption, shape stability, etc., the aboveresins are preferable. Particularly, polycarbonate is preferable. In theviewpoint of chemical resistance, low moisture absorption, etc.,amorphous polyolefin is preferable. In the viewpoint of high-speedresponsibility, etc., a glass substrate is preferable.

In order to give sufficient rigidity to the optical recording medium, itis preferable that the second substrate 27 is thick to some degree,having a thickness of not less than 0.3 mm. However, since a thinnersecond substrate 27 is more advantageous to make the recording/readingapparatus thinner, the thickness of the second substrate 27 ispreferably not greater than 3 mm, and more preferable not greater than1.5 mm.

An example of a preferable combination of the first substrate 21 and thesecond substrate 27 is that the first substrate 21 and the secondsubstrate 27 are made from the same material, and have the samethickness. By doing so, the rigidity of the first substrate 21 and thesecond substrate 27 are equivalent, which gives good balance. Whereby,the medium is prone not deform due to changes in environment, which ispreferable. In which case, it is preferable that the degrees anddirections of deformation of the both substrates caused by a change inenvironments are in harmony.

As another preferable example of the combination, the first substrate 21is as thin as about 0.1 mm, whereas the second substrate 27 is as thickas about 1.1 mm. By doing so, the objective lens can easily approach therecording layer, whereby the recoding density is easily increased.Accordingly, this is preferable. In this case, the first substrate 21may be in sheet-like shape, and may not have the guide groove.

A groove (guide groove) 32 used to guide a recording/reading light(recording/reading beam; for example, a laser beam) for recording orreading information is formed, spirally or concentrically, on the secondsubstrate 27. When the groove is formed on the second substrate 27 asthis, irregularities are formed on the surface of the second substrate27. The concave portion (groove) of the irregularities is called agroove, whereas the convex portion is called a land. Incidentally, thegroove 32 on the second substrate 27 is the convex portion with respectto the direction in which the light beam is irradiated.

Since the second recording layer 25 is formed in coating on thereflective layer 26 formed on the second substrate 27, the filmthickness of the second recording layer 25 is large at the groove(concave portion) on the second substrate 27 (this portion called athick film portion), whereas the film thickness of the same is small atthe 1 and (convex portion) on the second substrate 27 is small (thisportion called a thin film portion).

In this embodiment, recording or reading of information can be performedon the second recording layer 25 with the groove and/or land being asthe recording track because the depth of the groove 32 is small thereat,as will be described later.

In the case of the groove recording, the groove 32 on the secondsubstrate 27 is make slightly snake in the radial direction at apredetermined amplitude and a predetermined frequency to form wobble.Isolated pits (address pits) may be formed in the land between thegrooves 32 on the second substrate 27 (this called Land Pre-Pit; LPP),and address information may be beforehand recorded with the LandPre-Pits. Other concave or convex pits (pre-pits) may be formed asrequired.

According to this embodiment, in the case of the land recording, thegroove wall of the groove 32 on the second substrate 27 is made slightlysnake in the radial direction at a predetermined amplitude and apredetermined frequency to form wobble in the land. Address informationor other information may be beforehand recorded by forming pits in thegroove.

From the viewpoint of cost, it is preferable to manufacture the secondsubstrate 27 having such a concave portion and a convex portion ininjection molding with a stamper having a concave portion and a convexportion. When a resin layer made from a radiation-setting resin such asa photo-setting resin or the like is formed on the substrate made fromglass or the like, a concave portion and a convex portion such as arecording track and the like may be formed on the resin layer.

(i) With Respect to Buffer Layer 28

Here, the buffer layer 28 is provided as the intermediate layer betweenthe transparent adhesive layer 24 and the second recording layer 25.

The buffer layer 28 is to prevent two layers from dissolving in eachother and prevent the two layers from blending to each other. The bufferlayer 28 may have another function than the function of preventing thedissolving phenomenon. Further, still another intermediate layer may beput as required.

The material of the buffer layer 28 is required to be immiscible withthe second recording layer 25 or the transparent adhesive layer 24, andbe optically transmittable to some degree. The known inorganic ororganic material can be used for the buffer layer 28. In the viewpointof the properties, an organic material is preferably used. For example,(a) metal or semiconductor, (b) oxide, nitride, sulfide, trisulfide,fluoride or carbide of metal or semiconductor, and (c) amorphous carbonor the like are available. Among these, a layer made from an almosttransparent dielectric substance, or a very thin metal layer (includingalloy) is preferable.

In concrete, oxides such as silicon oxide, particularly, silicondioxide, zinc oxide, cerium oxide, yttrium oxide and the like; sulfidessuch as zinc sulfide, yttrium sulfide and the like; nitrides such assilicon nitride and the like; silicon carbide; a mixture (trisulfide) ofan oxide and sulfur; and alloys to be described later are preferable. Amixture of silicon oxide and zinc sulfide at a ratio of approximately30:70 to 90:10 by weight is preferable. A mixture (Y₂O₂S—ZnO) of sulfur,yttrium dioxide and zinc oxide is also preferable.

As the metal or alloy, silver or an alloy that contains silver as themain component and 0.1 to 15 atomic percent of at least one elementselected from the group consisting of titanium, zinc, copper, palladiumand gold is preferable. An alloy that contains silver as the maincomponent and 0.1 to 15 atomic percent of at least one rare earthelement is preferable, as well. As the rare earth element, neodymium,praseodymium, cerium or the like is preferable.

Alternatively, any resin layer can be used so long as it does not solvethe dye in the recording layer when the buffer layer is made.Particularly, a polymer film which can be fabricated in vacuumevaporation or CVD method is useful.

The thickness of the buffer layer 28 is preferably not less than 2 nm,and more preferably not less than 5 nm. When the buffer layer 28 isexcessively thin, prevention of the above mixing phenomenon tends to beinsufficient. The thickness of the buffer layer 28 is preferably notgreater than 2000 nm, and more preferably not greater than 500 nm.Excessively thick buffer layer is not only necessary for prevention ofthe mixing but also may cause a decrease in the optical transmission.When the layer is made from an inorganic substance, the film depositionof the layer takes a longer time, which causes a decrease inproductivity, or the film stress is increased. Thus, the film thicknessis preferably not greater than 200 nm. Particularly, since a film madefrom a metal excessively deteriorates the optical transmittance, thefilm thickness is preferably not greater than approximately 20 nm.

Another buffer layer may be provided as an intermediate layer betweenthe semitransparent reflective layer 23 and the transparent adhesivelayer 24, for example.

(j) With Respect to Other Layers

In this laminated structure, another layer may be arbitrarily put in thelayers as required. Alternatively, it is possible to arbitrarily provideanother layer on the outermost surface of the medium.

In concrete, it is possible to provide a protective layer to protect therecording layer or the reflective layer. The material of the protectivelayer is not specifically limited but any material is available so longas it protects the recording layer or the reflective layer from theexternal force. As an organic material of the protective layer,available are a thermal plastic resin, a thermal setting resin, anelectron beam setting resin, an ultraviolet ray-curable resin and thelike. As an organic material of the protective layer, available aresilicon oxide, silicon nitride, MgF₂, SnO₂ and the like.

The protective layer can be formed by dissolving a thermal plasticresin, a thermal setting resin or the like in an appropriate solvent toprepare a coating liquid, and applying and drying the liquid. In thecase of a ultraviolet ray-curable resin, the protective layer can beformed by preparing a coating liquid of the ultraviolet ray-curableresin itself or a coating liquid obtained by dissolving the ultravioletray-curable resin in an appropriate solvent, applying the coatingliquid, irradiating UV light to cure the liquid. As the ultravioletray-curable resins, available are acrylic resins such as urethaneacrylate, expoxy acrylate, polyester acrylate, etc. These materials canbe used solely or can be mixed to be used. Further, use of not only asingle layer but also a multilayer is possible.

As the method of forming the protective layer, there are coating methodssuch as spin coating, cast and the like, sputtering, chemicalevaporation, etc. Among these, spin coating is preferable.

The film thickness of the protective layer is generally within a rangefrom 0.1 to 100 μm. In the optical recording medium of this invention,the film thickness of the protective layer is preferably from 1 to 50μm.

Further, a print accepting layer, on which writing (printing) ispossible with various printers such as ink-jet printer, thermal printerand the like, or various writing tools, may be put on a surface that isnot a surface through which the recording/reading beam comes in, asrequired.

It is possible to add another recording layer in this structure torealize a structure having three or more recording layers. It is alsopossible to bond two optical recording media of this structure, with thefirst substrates 21 of the media being outside, to provide a largercapacity medium having four recording layers.

The single-sided incident type optical recording medium having the twodye containing recording layers 22 and 25 as above has the first dyecontaining recording layer 22 close to the side from which the lightbeam comes in (on one side), and the second dye containing recordinglayer 25 far from the same. For this, recording or reading ofinformation in and from the second dye containing recording layer 25positioned father from the side from which the light beam comes in areperformed by irradiating the light beam through the first dye containingrecording layer 22.

In such the single-sided incident type optical recording medium, whenthe depth of the groove (guide groove, concave portion) 32 formed on thesecond substrate 27 positioning on the opposite side to the side fromwhich the light beam comes in is, for example, approximately 150 nm likegeneral optical recording media, there is possibility that thereflectance necessary to perform recording or reading of information onthe second dye containing recording layer 25 cannot be obtained.

In the optical recording medium according to this embodiment, the depthof the groove 32 on the second substrate 27 is decreased to be shallowin a specific region, dissimilar to the depth of the groove of generaldye containing optical recording media so that a change in shape of thereflective layer reflecting the shape of the groove is made small.Whereby, it is possible to obtain sufficient reflectance to performrecording or reading of information on the second dye containingrecording layer 25. Such sufficiently large reflectance is advantageousto provide easy compatibility with DVD-ROM. When a shallow groove on thesecond substrate 27 is allowable, the productivity of the secondsubstrate 27 having the guide groove is improved, which improves themass productivity.

Unlike the known general dye optical recording media, the depth of thegroove 32 in a specific region on the second substrate 27 is madeshallow so that the reflectance sufficient to perform recording orreading of information on the second dye containing recording layer 25can be obtained. Whereby, it is possible to use the thin film portion25B or the thick film portion 25A of the second dye containing recordinglayer 25 as the recording track. Namely, the light beam can be emitted(irradiated) onto the land (convex portion) on the second substrate 27,that is, the concave portion (thin film portion 25B) of the secondrecording layer 27, to record or read information. Alternatively, thelight beam can be emitted (irradiated) onto the groove (concave portion)on the second substrate 27, that is, the convex portion (thick filmportion 25A) of the second recording layer 25, to record or readinformation.

In the optical recording medium of this embodiment, the depth of thegroove 32 on the second substrate 27 is practically set as below.

It is preferable that the depth (groove depth) of the groove 32 on thesecond substrate 27 is not less than 1/100×λ where λ represents therecording/reading wavelength. More preferably, the depth is not lessthan 2/100×λ, still more preferably 3/100×λ because the depth in thisdegree is preferable to secure sufficient reflectance and perform stabletracking.

When the recording/reading wavelength is λ=650 nm, for example, thedepth of the groove 32 on the second substrate 27 is preferably not lessthan 7 nm, more preferably not less than 13 nm, still more preferablynot less than 20 nm.

The depth of the groove 32 on the second substrate 27 is preferably notgreater than ⅙×λ, more preferably not greater than ⅛×λ, still morepreferably not greater than 1/10×λ because not excessively large depthis preferable to decrease a change in shape of the reflective layer inorder to secure the quantity of reflected light, and obtain highreflectance.

When the recording/reading wavelength is λ=650 nm, the depth of thegroove 32 on the second substrate 27 is preferably not greater than 108nm, more preferably not greater than 81 nm, still more preferably notgreater than 65 nm.

The width (groove width, G width; half width) of the groove 32 on thesecond substrate 27 is preferably not less than 1/10×T where Trepresents the track pitch, more preferably not less than 2/10×T, stillmore preferably not less than 3/10×T because excessively narrow groovewidth tends to make it difficult to easily track.

When the track pitch is 740 nm, for example, the width of the groove 32on the second substrate 27 is preferably not less than 74 nm, morepreferably not less than 148 nm, still more preferably not less than 222nm.

The width of the groove 32 on the second substrate 27 is preferably notgreater than 9/10×T, more preferably not greater than 8/10×T, still morepreferably not greater than 7/10×T because excessively wide groove tendsto make them difficult to easily track and to record well.

When the track pitch is 740 nm, for example, the width of the groove 32on the second substrate 27 is preferably not greater than 666 nm, morepreferably not greater than 592 nm, and still more preferably notgreater than 518 nm.

According to this embodiment, the depth of the groove 32 on the secondsubstrate 27 is smaller than the depth of the groove of the general dyeoptical recording medium, as above. It is preferable that the depth ofthe groove 32 on the second substrate 27 is smaller than the depth ofthe groove 31 on the first substrate 21.

When the recording/reading wavelength is 650 nm, for example, it ispreferable to set the depth of the groove 32 on the second substrate 27to not greater than 65 nm and set the depth of the grove 31 on the firstsubstrate 21 to not less than 108 nm. Note that a combination of thedepths of the grooves on the first substrate 21 and the second substrate27 to be set is not limited to the above example. It is only necessarythat the depth of the groove 32 on the second substrate 27 is smallerthan the depth of the groove 31 on the first substrate 21.

It is generally preferable that the depth of the groove 32 on the secondsubstrate 27 is smaller than the groove 31 on the first substrate 21.Specifically, the depth of the groove 32 on the second substrate 27 ispreferably not greater than 90% of the depth of the groove 31 on thefirst substrate 21, more preferably not greater than 80%, and still morepreferably not greater than 70%. The depth of the groove 32 on thesecond substrate 27 is generally not less than 5% of the depth of thegroove 31 on the first substrate 21, and preferably not less than 10%.

In the single-sided incident type optical recording medium as above,excellent recording/reading characteristics can be obtained when therecording track is formed in the concave portion (thick film portion22A) of the first dye containing recording layer 22. To the contrary,when the recording track is formed in the concave portion (thick filmportion 25A) of the second dye containing recording layer 25, there ispossibility that more excellent recording/reading characteristics (forexample, reflectance, polarity, maximum signal amplitude, etc.) cannotbe obtained.

In the optical recording medium according to this embodiment having thefirst dye containing recording layer 22 having the thick film portion22A and the thin film portion 22B, and the second dye containingrecording layer 25 having the thick film portion 25A and the thin filmportion 25B, the recording track is formed in the thick film portion 22Aof the first dye containing recording layer 22 closer to the side fromwhich the light beam comes in (on one side), whereas the recording trackis formed in the thin film portion 25B of the second dye containingrecording layer 25 farther from the side from which the light beam comesin, whereby more excellent recording/reading characteristics can beobtained.

The reason why more excellent recording/reading characteristics can beobtained when recording is performed on the thin film portion 25B of thesecond dye containing recording layer 25 is presumed that the followingfact influences.

The phase difference (difference in optical path length) between theconcave portion (non-track portion) and the convex portion (trackportion) at the time that the light beam is irradiated is important inorder to perform tracking on the recording track to record or read well.

In the first recording layer 22, a difference between the reflectedlight beam from an interface between the first recording layer 22 andthe semitransparent reflective layer 23 at the concave portion and thereflected light beam from an interface between the same at the convexportion is equivalent to a difference in optical path length. Thedifference in optical path length is determined from, mainly, a distanced1 between surfaces of the concave portion and the convex portion of thefirst recording layer 22 on the side from which the light beam comes in(a distance between a surface of the thin film portion 22B of the firstrecording layer 22 and a surface of the thick film portion 22A of thesame on the first substrate's side) (refer to FIG. 1), a difference infilm thickness between the concave portion and convex portion of thefirst recording layer 22, a complex index of refraction of the firstrecording layer 22, and a complex index of refraction of the firstsubstrate 21.

On the other hand, in the second recording layer 25, a differencebetween the reflected light beam from an interface at the concaveportion between the second recording layer 25 and the reflective layer26 and the reflected light beam from an interface at the convex portionbetween the same is equivalent to a difference in optical path length.The difference in optical path length is mainly determined from adistance d2 between surfaces of the concave portion and convex portionof the second recording layer 25 on the side from which the light beamcomes in [a distance between a surface of the thin film portion 25B ofthe second recording layer 25 and a surface of the thick film portion25A of the same on the transparent adhesive layer's side (intermediatelayer's side)] (refer to FIG. 1), a difference in film thickness betweenthe concave portion and convex portion of the second recording layer 25,a complex index of refraction of the second recording layer 25, and acomplex index of refraction of the transparent adhesive layer 24.

In this case, d2 inevitably differs from d1. Namely, since the groove isfilled to some degree by coating the recording layer thereon and theirregularity is formed on the surface in such state, d2 is considerablysmaller than d1.

For this, the difference in optical path length and the difference inphase in turn behave differently from those of the first recording layer22. For this reason, it is presumed that the recording in the thin filmportion 25B is more preferable in the second recording layer 25.

In order to form the recording track in the thin film portion 25B of thesecond recording layer 25, it is necessary to secure the thicknessnecessary to allow the thin film portion 25B of the second recordinglayer 25 to function as the recording layer. Namely, it is preferablethat the thin film portion 25B of the second recording layer 25 has athickness (L film thickness) not less than predetermined thickness (forexample, 70 nm). When the second recording layer 25 is formed by coatinga solvent containing a dye in spin coating, for example, the filmthickness of the second recording layer 25 can be not less thanpredetermined thickness by varying the concentration of the dye orvarying the number of spin rotation as the recording layer coatingcondition.

The difference in film thickness between the thick film portion 25A andthe thin film portion 25B of the second recording layer 25 is preferablynot less than 1/100×λ/n where λ represents the recording/readingwavelength and n represents the refractive index of the second recordinglayer 25, more preferably not less than 2/100×λ/n, and still morepreferably not less than 3/100×λ/n.

The difference in film thickness is preferably not greater than ⅓×λ/n,more preferably not greater than ¼×λ/n, and still more preferably notgreater than ⅕×λ/n.

Practically, when the recording/reading wavelength is λ=650 nm and therefractive index of the second recording layer 25 is n=2.2, thedifference in film thickness between the thick film portion 25A and thethin film portion 25B of the second recording layer 25 is preferably notless than 3 nm, more preferably not less than 6 nm, and still morepreferably not less than 9 nm. The difference in film thickness ispreferably not greater than 98 nm, more preferably not greater than 74nm, and still more preferably not greater than 59 nm.

On the other hand, the difference in film thickness between the thickfilm portion 22A and the thin film portion 22B of the first recordinglayer 22 is preferably not less than 1/30×(λ/n) where λ represents therecording/reading wavelength and n represents the refractive index ofthe first recording layer, more preferably not less than 2/30×λ/n, andstill more preferably not less than 3/30×λ/n.

The difference in film thickness is preferably not greater than4/4×(λ/n), more preferably not greater than ⅘×(λ/n), and still morepreferably not greater than 4/6×(λ/n).

Practically, when the recording/reading wavelength is λ=650 nm and therefractive index of the first recording layer 22 is n=2.2, thedifference in film thickness between the thick film portion 22A and thethin film portion 22B of the first recording layer 22 is preferably notless than 10 nm, more preferably not less than 20 nm, and still morepreferably not less than 30 nm. The difference in film thickness ispreferably not greater than 295 nm, more preferably not greater than 236nm, and still more preferably not greater than 197 nm.

In this embodiment, since the thick film portion 22A and the thin filmportion 22B of the first recording layer 22 are formed correspondinglyto the concave portion and the convex portion on the first substrate 21positioned on the side from which the light beam comes in, it ispreferable that the recording track is formed in the groove (concaveportion) on the first substrate 21, that is, in the convex portion(thick film portion 22A) of the first recording layer 22 protruding tothe side from which the light beam comes in.

In this case, recording or reading of information is performed byemitting (irradiating) the light beam to the groove (concave portion) onthe first substrate 21, that is, the convex portion (thick film portion22A) of the first recording layer 22, in this optical recording medium.

In this embodiment, since the thick film portion 25A and the thin filmportion 25B of the second recording layer 25 are formed correspondinglyto the concave portion and the convex portion on the second substrate 27positioned on the opposite side to the side from which the light beamcomes in, it is preferable that the recording track is formed in theland (convex portion) on the second substrate 27, that is, the convexportion (thin film portion 25B) of the second recording layer 25protruding to the side from which the light beam comes in.

In this case, recording or reading of information is performed byemitting (irradiating) the light beam to the land (convex portion) onthe second substrate 27, that is, the convex portion (thin film portion25B) of the second recording layer 25 in the optical recording medium.

In the optical recording medium according to this invention, therecording track may be formed in the groove on the first substrate 21,whereas the recording track may be formed in the land on the secondsubstrate 27. In such case, it may be necessary to change the trackingpolarity when information is recorded in or read from the recordinglayers.

According to this embodiment, the depth of the groove 32 on the secondsubstrate 27 is set smaller than the depth of the groove of the knowndye optical recording medium, or the recording track is formed in thethin film portion 25B of the second dye containing recording layer 25farther from the side from which the light beam comes in, whereby theinformation recording/reading characteristics of the second dyecontaining recording layer 25 are improved. It is more preferable tocombine the above manners so that the depth of the groove 32 on thesecond substrate 27 is set shallower and the recording track is formedin the thin film portion 25B of the second dye containing recordinglayer 25.

Whereby, it is possible to stably obtain sufficient reflectance forrecording or reading information in or from the dye containing recordinglayer 25, and more excellent recording characteristics.

(2) Optical Recording Medium Manufacturing Method

Next, description will be made of a method of manufacturing the opticalrecording medium structured as above.

First, a laminated structure (first information recording body) havingthe first recording layer 22 containing a dye and the semitransparentreflective layer 23 in this order on the transparent first substrate 21is fabricated. On the other hand, a laminated structure (secondinformation recording body) having the reflective layer 26, the secondrecording layer 25 containing a dye and the buffer layer 28 in thisorder on the second substrate 27 is fabricated. The first informationrecording body and the second information recording body are bondedtogether through the transparent adhesive layer 24, with the recordinglayers facing to each other.

Practically, the transparent first substrate 21, on the surface of whichgrooves, lands and pre-pits are formed as a concave portion and a convexportion, is fabricated in injection molding, 2P method or the like(method of transferring a concave portion and a convex portion with aresin stamper having a concave portion and a convex portion to a settingresin such as a photo-setting resin or the like, and hardening the resinto make the substrate).

Next, at least an organic dye is dissolved in a solvent, deposited on asurface having the concave portion and the convex portion of the firstsubstrate 21 in spin coating or the like to form the first recordinglayer 22.

On the first recording layer 22, an Ag alloy or the like is spattered orevaporated to deposit the semitransparent reflective layer 23, therebyto fabricate the first information recording body.

Next, the second substrate 27, on a surface of which grooves, lands andpits are formed as a concave portion and a convex portion, is fabricatein injection molding, 2P method or the like. An Ag alloy or the like isspattered or evaporated on a surface having the concave portion and theconvex portion of the second substrate 27 to deposit the reflective filmlayer 26.

Further, at least an organic dye is dissolved in a solvent, deposited inspin coating or the like to form the second recording layer 25.

Next, a dielectric material or the like is spattered to deposit thebuffer layer 28, thereby to fabricate the second information recordingbody.

An adhesive of a ultraviolet ray-curable resin or the like is coated onthe first information recording body, the second information recordingbody is mounted thereon, and the adhesive is spread over the entiresurface thereof by, for example, rotating it at high speed or applyingpressure. This is performed while adjusting the speed or pressure sothat the film thickness of the adhesive layer is in a predeterminedrange.

After that, ultraviolet ray is irradiated from the first informationrecording body's side through the semitransparent reflective layer 23 toharden the adhesive of an ultraviolet ray-curable resin or the like toadhere the recording bodies together, whereby the optical recordingmedium is fabricated.

Alternatively, the ultraviolet ray may be irradiated from the side ofthe media. In either case, care should be taken not to damage the dyerecording layer by the ultraviolet ray. It is possible to use apressure-sensitive double-sided adhesive tape, put the tape between thefirst information recording body and the second information recordingbody, and press them to form an adhesive layer. Alternatively, theadhesive layer can be formed by using an adhesive of the delayed-curabletype, coating the adhesive on the first information recording body inscreen printing or the like, irradiating ultraviolet ray thereon,mounting the second information recording body on the first informationrecording body, and pressing them. Generally, many of the adhesives ofthe delayed-curable type are opaque.

Next, description will be made of a method of fabricating the substrates21 and 27 having guide grooves (a concave portion and a convex portion).

For example, to form a concave portion and a convex portion (grooves) onthe substrates 21 and 27, a metal stamper having desired a concaveportion and a convex portion is used, and the concave portion and theconvex portion are transferred onto a resin material in injectionmolding to make the first substrate 21. Another stamper having reverse aconcave portion and a convex portion is used, the concave portion andthe convex portion are transferred onto a resin material in injectionmolding to make the second substrate 27.

For example, wobble is particularly formed on the recording track togive synchronization information, address information and the likethereto.

In this embodiment, since the recording track is formed in the thickfilm portion 22A of the first recording layer 22, whereas the recordingtrack is formed in the thin film portion 25B of the second recordinglayer 25, the wobble is formed on the concave portion on the firstsubstrate 21 and the convex portion on the second substrate 27.

The wobble is formed on the concave portion on the first substrate 21 inthe following procedure.

First, a glass substrate/photo-resist is exposed and developed while thebeam is made meander to obtain a negative having a concave portion and aconvex portion. On the negative having a concave portion and a convexportion, the wobble is generally formed on the concave portion (groove).

A stamper is made, using the negative having a concave portion and aconvex portion. With the made stamper, the first substrate 21 having aconcave portion and a convex portion (grooves, guide grooves) is made ininjection molding. In this case, since the wobble is formed on theconvex portion of the stamper, the wobble is made on the concave portionon the first substrate 21.

To form the wobble on the convex portion on the second substrate 27, itis necessary that the wobble is present on the concave portion of thestamper. For this, the above method cannot give the wobble to the convexportion on the second substrate 27.

So, a stamper is made in the same manner as the stamper used to form theconcave portion and the convex portion (grooves, guide grooves) on thefirst substrate 21 is made. Here, it is necessary to change the shape ofthe concave portion and the convex portion (groove depth, groove width,width of meandering, etc.) to match it to a concave portion and a convexportion to be formed on the second substrate 27.

Next, the concave portion and the convex portion are transferred fromthe stamper to make a negative stamper having reverse a concave portionand a convex portion. In this case, since the wobble is formed on theconvex portion on the stamper, the wobble is formed on the concaveportion on the negative stamper.

With the negative stamper, the second substrate 27 having the concaveportion and the convex portion (grooves, guide grooves) is fabricated ininjection molding. In this case, since the wobble is formed on theconcave portion on the negative stamper, the wobble is formed on theconvex portion on the second substrate 27.

(3) Optical Recording Medium Recording/Reading Method

Next, description will be made of a recording/reading method for theoptical recording medium according to this embodiment.

Recording in the optical recording medium structured as above isperformed by irradiating a laser beam focused to a diameter ofapproximately 0.5 to 1 μm onto the recording layer from the firstsubstrate's side. In a portion on which the laser beam is irradiated,thermal deformation of the recording layer such as decomposition, heatbuild-up, dissolution or the like occurs due to absorption of the energyof the laser beam, the optical characteristics of the recording layerare thereby changed.

Reading of recorded information is performed by reading, with the laserbeam, a difference in reflectance between a portion in which the opticalcharacteristics have changed and a portion in which the opticalcharacteristics remain unchanged.

Recording and reading are performed on each of the two recording layersin the following manner. Whether the converging position of theconverged laser beam is on the first recording layer 22 or the secondrecording layer 25 can be discriminated by using a focus error signalobtained in the knife edge method, astigmatism method, Foucault methodor the like. Namely, when the objective lens for focusing the laser beamis shifted in the vertical direction, a different S-shaped curve isobtained according to whether the focus position of the laser beam is onthe first recording layer 22 or on the second recording layer 25. It ispossible to select the first recording layer 22 or the second recordinglayer 25 to be recorded or read by selecting either one of the S-shapedcurves.

In this embodiment, when information is recorded or read in or from thefirst recording layer 22, it is preferable that the light beam isemitted (irradiated) on the groove (concave portion) on the firstsubstrate 21, that is, the convex portion (thick film portion 22A) ofthe first recording layer 22, to record or read information. Wheninformation is recorded or read in or from the second recording layer25, it is preferable that the laser beam is emitted (irradiated) on theland (convex portion) on the second substrate 27, that is, the convexportion (thin film portion 25B) of the second recording layer 25, torecord or read information.

As the laser beam used for the optical recording medium according tothis embodiment, N₂, He—Cd, Ar, He—Ne, ruby, semiconductor, dye laser,etc. are available. Among these, the semiconductor laser is preferablebecause of its light weight, compactness, facility, etc.

It is preferable that the wavelength of the used laser beam is as shortas possible for the purpose of high-density recording. Particularly, thelaser beam having a wavelength of 350 to 530 nm is preferable. As atypical example of such laser beam, there are laser beams having centerwavelengths of 405 nm, 410 nm and 515 nm. For example, the laser beamhaving a wavelength within a range from 350 to 530 nm can be obtained byusing a 405 nm or 410 blue high-power semiconductor laser or a 515 nmbluish green high-power semiconductor laser. Other than these, the laserbeam can be obtained by wavelength-modulating, by means of a secondharmonic generating element (SHG), either (a) a semiconductor laser thatcan continuously oscillate fundamental oscillation wavelengths of 740 to960 nm, or (b) a solid state laser that is excited by a semiconductorlaser and can continuously oscillate fundamental oscillation wavelengthsof 740 to 960 nm.

As the above SHG, any piezo element lacking inversion symmetry isusable, but KDP, ADP, BNN, KN, LBO and compound semiconductors arepreferable. As practical examples of the second harmonic wave, there are430 nm which is a double of 860 nm in the case of a semiconductor laserhaving a fundamental oscillation wavelength of 860 nm, 430 nm which is adouble of 860 nm from Cr-doped LiSrAlF₆ crystal (having a fundamentaloscillation wavelength of 860 nm) in the case of a solid laser excitedby a semiconductor laser, etc.

(4) Working/Effects

According to the optical recording medium, and the manufacturing methodfor the optical recording medium and the recording/reading method forthe optical recording medium of this embodiment, in an optical recordingmedium having a plurality of recording layers 22 and 25 in whichinformation is recorded or read by irradiating a laser beam from oneside thereof, the laser beam is irradiated on the thin film portion 25Bof the second recording layer 25 positioned farther from the side fromwhich the laser beam comes in to perform recording or reading ofinformation. It is thus possible to obtain sufficient reflectance andexcellent recording/reading characteristics (for example, reflectance,polarity, maximum signal amplitude, etc.) when recording or reading ofinformation is performed on the second recording layer 25 positionedfarther from the side from which the laser beam comes in. As a result,excellent recording/reading characteristics can be obtained in both theplural recording layers 22 and 25.

(5) Others

In this embodiment, this invention is applied to a single-sided incidenttype DVD-R of the bonded dual layer type. However, this invention is notlimited to this. This invention can be applied to an optical recordingmedium in any structure so long as it has dye containing recordinglayers in which recording or reading of information is performed byirradiating a laser beam from one side thereof.

As shown in FIG. 2, this invention can be applied to a single-sidedincident type DVD-R of the laminated dual layer type having a firstrecording layer containing a dye (first dye containing recording layer)2, a semitransparent reflective layer (semitransparent reflective layer)3, an intermediate resin layer (intermediate layer) 4, a secondrecording layer containing a dye (second dye containing recording layer)5, a reflective layer 6, a second substrate 78 (comprised of an adhesivelayer 7 and a substrate body 8) in this order on a disk-shapedtransparent (light-transmissible) first substrate (firstlight-transmissible substrate) 1. Incidentally, reference characters 11and 12 designate guide grooves (grooves, concave portions).

In this case, recording or reading information in or from the secondrecording layer 5 positioning farther from the side from which the laserbeam comes in is performed with a guide groove (groove, concave portion)12 formed on the second substrate 78 (substrate positioning on theopposite side to the side from which the laser beam comes in). In orderto obtain sufficient reflectance, the depth of the guide groove 12 iswithin a range from 1/100×λ to ⅙×λ where λ represents therecording/reading wavelength.

In order to obtain excellent recording/reading characteristics, it ispreferable to form the recording track in the groove (groove, concaveportion) 12 on the second substrate 78, that is, the concave portion(thin film portion) of the second recording layer 5. Namely, it ispreferable to emit (irradiate) the laser beam on the groove (groove,concave portion) 12 on the second substrate 78, that is, the concaveportion (thin film portion) of the second recording layer 5 to record orread information.

When this invention is applied to a single-sided incident type opticalrecording medium of the bonded dual layer type, this invention isparticularly effective and preferable. Namely, it is effective that thisinvention is applied to an optical recording medium which has a firstinformation recording body having at least a first dye containingrecording layer containing a first dye and a semitransparent reflectivelayer laminated in this order on a first substrate having a guide grooveand a second information recording body having at least a reflectivelayer and a second dye containing recording layer containing a seconddye laminated in this order on a second substrate having a guide groove,the optical recording medium being formed by bonding the firstinformation recording body and the second information recording bodytogether through an optically transparent adhesive layer, with surfaceson the opposite side to the substrates of the first informationrecording body and the second information recording body facing to eachother, in which information is recorded or read by irradiating a laserbeam from the first substrate's side.

Since two information recording bodies are bonded together, with theopposite sides facing to each other, in such optical recording medium, astate in which the grooves are filled with the dye containing recordinglayer, or a difference in optical path length between the groove and theland tends to differ between the two information recording bodies. Forthis, it is considered that the optimum depth of the groove on thesecond substrate differs from that on the first substrate, and a smallerdepth of the groove on the second substrate than the depth of the grooveon the first substrate can yield the optimum value.

This invention can be applied to not only an optical recording medium ofa so-called substrate incident type but also an optical recording mediumof a so-called film incident type. Namely, this invention can be appliedto an optical recording medium (optical recording medium having one dyecontaining recording layer) having a substrate body (including aprotective layer, substrate, etc.), a dye containing recording layer,reflective layer and a substrate having a guide groove, in whichinformation is recorded or read by irradiating a light beam from thesubstrate body's side (opposite side to the substrate).

In such case, in order to obtain sufficient reflection from the guidegroove (groove, concave portion) formed on the substrate (substratepositioning on the opposite side to the side from which the light beamcomes in), the depth of the groove on the substrate is within a rangefrom 1/100×λ to ⅙×λ where λ represents the recording/reading wavelength.

Whereby, sufficient reflectance can be obtained when information isrecorded in or read from the dye containing recording layer byirradiating the light beam from the opposite side to the substrate. As aresult, excellent recording/reading characteristics are obtained.

Note that this invention is not limited to the above embodiment, but maybe modified in various ways without departing from the scope of theinvention.

EXAMPLES

Next, this invention will be further described in detail by way ofexamples. Note that this invention is not limited to the followingexamples.

(Preparation of Optical Recording Medium)

An optical recording medium in examples and comparative examples has afirst information recording body having at least a first dye containingrecording layer containing a first dye and a semitransparent reflectivelayer laminated in this order on a first substrate having a guidegroove, and a second information body having at least a reflective layerand a second dye containing recording layer containing a second dyelaminated in this order on a second substrate 2 having a guide groove,and is formed by bonding the first information recording body and thesecond information recording body through an optically transparentadhesive layer, with the opposite sides to the substrates of the firstinformation recording body and the second information recording bodyfacing to each other.

Hereinafter, preparation of the second information recording body willbe mainly described.

First, a second substrate (having a refractive index of 1.56) made ofpolycarbonate having a thickness of 0.6 mm, which had a track pitch of740 nm and a guide groove having predetermined depth and predeterminedwidth was prepared in injection molding, with a mother stamper (negativestamper).

Next, a silver alloy containing not less than 97 atomic percent of Agwas spattered on the second substrate to form a reflective layer.

On the reflective layer, a octafluoropentanol solution of a metalcomplex azo dye was spin-coated under a predetermined coating condition(dye concentration), dried at a temperature of 100° C. for 30 minutes toform a second dye containing recording layer. Here, the coatingcondition was changed to give predetermined film thickness to the seconddye containing recording layer. This recording layer had a refractiveindex of 2.25 and an extinction coefficient of 0.02.

On the second dye containing recording layer, any one of a silver alloycontaining not less than 97 atomic percent of Ag, (ZnS)₈₀ (SiO₂)₂₀ orSiO₂ was spattered to form a buffer layer, an ultraviolet ray-curableresin (SPC-920 manufactured by Nippon Kayaku Co., Ltd.) was spin-coatedthereon to a thickness of approximately 5 to 7 μm to form a protectivelayer.

Generally, a radical type ultraviolet ray-curable resin (adhesive) iscoated in spin coating on the above protective layer, bonded to a firstsubstrate containing a recording layer (first recording layer)separately prepared, with this layer and the reflective layer of thefirst substrate facing to each other, to prepare an optical recordingmedium.

Here, in order to eliminate an effect of the first information recordingbody and precisely evaluate the characteristics of the second recordinglayer, a groove-less polycarbonate substrate (having a refractive indexof 1.56) having a thickness of 0.6 mm without the recording layer andthe semitransparent reflective layer was used as the first informationrecording body. The refractive index of the adhesive layer afterhardened was 1.53.

(Measuring Method)

First, the reflectance of the un-recorded second recording layer wasmeasured by an evaluation machine equipped with a semiconductor laserhaving a wavelength of 657 nm (NA=0.65) (DDU-1000 having the maximumrecording power of 15 mW manufactured by Pulstec Industrial Co., Ltd).Next, EFM+ signals modulated in 8/16 modulation were recorded at arecording linear velocity of 3.8 m/s (1-times velocity) with a recordingpower at which the asymmetry of the recorded signals was approximatelyzero, and the reflectance, the polarity and the maximum signal amplitude(amplitude of the longest mark; so-called Modulation; I14/I14H) of therecorded signals were measured.

In these examples, it was considered that a reflectance of not less than25% is excellent, and a reflectance of 30% is more excellent. Forkeeping the compatibility with DVD-ROM, a reflectance of several tens %at the unrecorded portion of the second recording layer is sufficient,in general. Since the media in these examples do not have the firstrecording layer and the semitransparent reflective layer, there istendency that the reflectance is higher than that of practical examples.When a reflectance of not less than 25% is obtained in these examples,it is considered that a reflectance of several tens % can be obtainedeven if practical effects of the first information recording body areconsidered.

For the compatibility with DVD-ROM or the like, it is desirable that thepolarity of the recorded signals is H to L.

It is generally preferable that the maximum signal amplitude is large.The maximum signal amplitude is generally preferably not less than 0.5,and more preferably not less than 0.6. However, the maximum signalamplitude can be improved by adjusting the film thickness of therecording layer, the groove shape, the recording power, the recordingpulse waveform (recording strategy), etc. For this, even when the valueof the maximum signal amplitude is small, the medium is usable as anoptical recording medium only if the reflectance is sufficient. Forexample, the maximum signal amplitude can be increased by recording thesignals at high recording power to increase the width of the recordingmark in the direction of the track.

Results of measurement of reflectance, polarity, maximum signalamplitude in the examples and comparative examples are as shown in Table1 below.

To satisfy a relationship of the groove depth of 1/100×λ to ⅙×λ at thetime that the recording/reading wavelength is λ=657 nm, the groove depthis required to fall within a range from 6.57 nm to 109.5 nm.

TABLE 1 recording Layer coating conditions groove L G L film G film Max.(dye recording depth width width thickness thickness buffer signalconcentration; portion (nm) (nm) (nm) (nm) (nm) layer reflectancepolarity amplitude wt %) Ex. 1 G 65 420 320 70 85 Ag alloy 30.7 HtoL<0.1 3.55 wt % Ex. 2 L 65 420 320 70 85 Ag alloy 40.0 HtoL 0.79 3.55 wt% Ex. 3 L 65 420 320 70 85 SiO2 28.1 — — 3.55 wt % Ex. 4 L 65 420 320 80100 Ag alloy 27.4 — — 4.43 wt % Ex. 5 L 65 420 320 80 100 ZnS—SiO2 26.7— — 4.43 wt % Ex. 6 L 65 420 320 80 100 SiO2 29.7 — — 4.43 wt % Ex. 7 G50 330 410 75 105 Ag alloy 40.1 — — 3.55 wt % Ex. 8 G 50 330 410 75 105SiO2 30.9 HtoL 0.36 3.55 wt % Ex. 9 G 50 330 410 95 130 Ag alloy 29.1HtoL LS 4.43 wt % Ex. 10 G 50 330 410 95 130 ZnS—SiO2 31.3 HtoL 0.314.43 wt % Ex. 11 G 50 330 410 95 130 SiO2 31.2 HtoL 0.3 4.43 wt % Ex. 12L 50 330 410 75 105 Ag alloy 45.3 — — 3.55 wt % Ex. 13 L 50 330 410 75105 ZnS—SiO2 29.0 — — 3.55 wt % Ex. 14 L 50 330 410 75 105 SiO2 36.4HtoL 0.74 3.55 wt % Ex. 15 L 50 330 410 95 130 Ag alloy 31.1 HtoL 0.714.43 wt % Ex. 16 L 50 330 410 95 130 ZnS—SiO2 38.2 HtoL 0.66 4.43 wt %Ex. 17 L 50 330 410 95 130 SiO2 36.1 HtoL 0.73 4.43 wt % Ex. 18 G 30 520220 70 110 Ag alloy 43.0 — — 3.10 wt % Ex. 19 G 30 520 220 70 110ZnS—SiO2 39.0 HtoL 0.22 3.10 wt % Ex. 20 G 30 520 220 70 110 SiO2 43.2HtoL 0.11 3.10 wt % Ex. 21 G 30 520 220 90 135 Ag alloy 30.2 HtoL <0.13.55 wt % Ex. 22 G 30 520 220 90 135 ZnS—SiO2 41.7 — — 3.55 wt % Ex. 23G 30 520 220 90 135 SiO2 38.9 HtoL 0.17 3.55 wt % Ex. 24 L 30 520 220 70110 Ag alloy 49.1 — — 3.10 wt % Ex. 25 L 30 520 220 70 110 ZnS—SiO2 43.9HtoL 0.72 3.10 wt % Ex. 26 L 30 520 220 70 110 SiO2 49.5 HtoL 0.58 3.10wt % Ex. 27 L 30 520 220 90 135 Ag alloy 33.9 HtoL 0.79 3.55 wt % Ex. 28L 30 520 220 90 135 ZnS—SiO2 47.9 — — 3.55 wt % Ex. 29 L 30 520 220 90135 SiO2 44.2 HtoL 0.55 3.55 wt % Compar. G 120 410 330 30 70 Ag alloy9.0 — — 1.90 wt % Ex. 1 Compar. G 120 410 330 30 70 SiO2 6.5 — — 1.90 wt% Ex. 2 Compar. G 160 430 310 20 75 Ag alloy 12.9 — — 1.90 wt % Ex. 3Compar. G 160 430 310 20 75 SiO2 19.9 — — 1.90 wt % Ex. 4

Example 1

In Example 1, on the second substrate, a guide groove was formed to havea groove depth of 65 nm (corresponding to approximately λ/10), a groovewidth (G width) of 320 nm and a land width (L width) of 420 nm.

The buffer layer was formed by spattering an Ag alloy. A metal complexazo dye having a dye concentration of 3.55 wt % as a coating conditionwas spin-coated to form the second recording layer.

The film thickness (thick film portion, G film thickness) of the grooveportion of the second recording layer formed as above was 85 nm, and thefilm thickness (thin film portion, L film thickness) of the land portionwas 70 nm.

The reflectance at the groove portion of the optical recording mediumprepared as above was measured under the above conditions. Thereflectance was 30.7%, as shown in Table 1.

The polarity and the maximum signal amplitude of the recorded signalswere measured. The polarity of the recorded signals was H to L, and themaximum signal amplitude thereof was smaller than 0.1 (when recorded ata recording power of 15 nW), as shown in Table 1. However, since themaximum signal amplitude can be improved by adjusting the film thicknessand the like of the recording layer, it is considered that an opticalrecording medium as this is usable.

Example 2

In Example 2, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 1 wasmeasured under the above conditions. As a result, the reflectance was40.0%, as shown in Table 1.

The polarity and the maximum signal amplitude of the recorded signalswere measured. As shown in Table 1, the polarity of the recoded signalswas H to L, and the maximum signal amplitude of the same was 0.79.

As above, it was found that reflectance necessary for recording/readingcan be obtained in the land recording instead of the groove recording.

It was found that, even in an optical recording medium prepared in thesame manner as Example 1, characteristics necessary forrecording/reading can be obtained when the land recording is performedlike Example 2.

Example 3

In Example 3, the reflectance was measured in the same manner as theabove Example 2 excepting that the material of the buffer layer wasSiO₂. The reflectance was 28.1%, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to SiO₂.

Example 4

In Example 4, the reflectance was measured in the same manner as theabove Example 2 excepting that the dye concentration as being thecoating condition was 4.43 Wt %, whereby the film thickness (thick filmportion, G film thickness) of the groove portion of the second recordinglayer was 100 nm, and the film thickness (thin film portion, L filmthickness) of the land portion of the same was 80 nm (namely, adifference in film thickness between the thick film portion and the thinfilm portion of the second recording layer was 20 nm). The reflectancewas 27.4%, as shown in Table 1.

As this, it was found that reflectance necessary for recording/readingcan be obtained even when the film thickness of the second recordinglayer is varied.

Example 5

In Example 5, the reflectance was measured in the same manner as theabove Example 4 excepting that the material of the buffer layer wasZnS—SiO₂. The reflectance was 26.7%, as shown in Table 1.

As this, it was found that reflectance necessary for recording/readingcan be obtained even when the material of the buffer layer is changedfrom an Ag alloy to ZnS—SiO₂.

Example 6

In Example 6, the reflectance was measured in the same manner as theabove Example 3 excepting that the dye concentration as being thecoating condition was 4.43 wt %, whereby the film thickness (thick filmportion, G film thickness) of the groove portion of the second recordinglayer was 100 nm, and the film thickness (thin film portion, L filmthickness) of the land portion of the same was 80 nm (namely, adifference in film thickness between the thick film portion and the thinfilm portion of the second recording layer was 20 nm). The reflectancewas 29.7%, as shown in Table 1.

As this, it was found that reflectance necessary for recording/readingcan be obtained even when the film thickness of the second recordinglayer is varied.

Example 7

In Example 7, the guide groove was formed on the second substrate tohave the groove depth of 50 nm (corresponding to approximately λ/13),the groove width (G width) of 410 nm, and the land width (L width) of330 nm.

An Ag alloy was spattered to form the buffer layer. A metal complex azodye having a dye concentration of 3.55 wt % as the coating condition wasspin-coated to form the second recording layer.

The film thickness (thick film portion, G film thickness) of the grooveportion of the second recording layer prepared as this was 105 nm, andthe film thickness (thin film portion, L film thickness) of the landportion of the same was 75 nm.

The reflectance at the groove portion of an optical recording mediumprepared as this was measured under the above conditions. Thereflectance was 40.1%, as shown in Table 1.

As this, it was found that the reflectance increases as the depth of thegroove on the second substrate decreases, as compared with the aboveExample 1.

Example 8

In Example 8, the reflectance was measure in the same manner as theabove Example 7 excepting that the material of the buffer layer wasSiO₂. The reflectance was 30.9%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.36. Since the maximum signalamplitude can be improved by adjusting the film thickness and the likeof the recording layer, it is considered that an optical recordingmedium as this is usable.

As this, it was found that reflectance necessary for recording/readingcan be obtained even when the material of the buffer layer is changedfrom an Ag alloy to SiO₂.

Example 9

In Example 9, the reflectance was measured in the same manner as theabove Example 7 excepting that the dye concentration as being thecoating condition was 4.43 wt %, whereby the film thickness (thick filmportion, G film thickness) of the groove portion of the second recordinglayer recording was 130 nm, and the film thickness (thin film portion, Lfilm thickness) of the land portion of the same was 95 nm (namely, adifference in film thickness between the thick film portion and the thinfilm portion of the second recording layer was 35 nm). The reflectancewas 29.1%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was LS (failing to record due topoor sensitivity; when recording at a recording power of 15 mW).However, since the maximum signal amplitude can be improved by adjustingthe film thickness and the like of the recording layer, it is consideredthat an optical recording medium as this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the film thickness of the second recording layer isvaried.

Example 10

In Example 10, the reflectance was measured in the same manner as theabove Example 9 excepting that the material of the buffer layer wasZnS—SiO₂. The reflectance was 31.3%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.31, as shown in Table 1.However, since the maximum signal amplitude can be improved by adjustingthe film thickness and the like of the recording layer, it is consideredthat an optical recording medium as this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to ZnS—SiO₂.

Example 11

In Example 11, the reflectance was measured in the same manner as theabove Example 8 excepting that the dye concentration as being thecoating condition was 4.43 wt %, whereby the film thickness (thick filmportion, G film thickness) of the groove portion of the second recordinglayer was 100 nm, and the film thickness (thin film portion, L filmthickness) of the land portion of the same was 80 nm (that is, adifference in film thickness between the thick film portion and the thinfilm portion of the second recording layer was 20 nm). The reflectancewas 31.2%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.3, as shown in Table 1.However, since the maximum signal amplitude can be improved by adjustingthe film thickness and the like of the recording layer, it is consideredthat an optical recording medium as this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the film thickness of the second recording layer isvaried.

Example 12

In Example 12, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 7 wasmeasured under the above conditions. As a result, the reflectance was45.3%, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

Example 13

In Example 13, the reflectance was measured in the same manner as theabove Example 12 excepting that the material of the buffer layer wasZnS—SiO₂. The reflectance was 29.0%, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to ZnS—SiO₂.

Example 14

In Example 14, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 8 wasmeasured under the above conditions. As a result, the reflectance was36.4%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.74, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that excellent recording/reading characteristics canbe obtained even when the material of the buffer layer is SiO₂, and thefilm thickness and groove shape of the second recording layer arevaried.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whengroove recording is performed like Example 8. To the contrary, it wasfound that the characteristics necessary for recording/reading can beobtained when the land recording is performed like this Example 14.

Example 15

In Example 15, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 9 wasmeasured. As a result, the reflectance was 31.1%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.71, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that excellent recording/reading characteristics canbe obtained even when the material of the buffer layer is changed fromSiO₂ to an Ag alloy, and the film thickness of the second recordinglayer is varied.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whenthe groove recording is performed like Example 9. To the contrary, itwas found that characteristics necessary for recoding/reading can beobtained when the land recording is performed like this Example 15.

Example 16

In Example 16, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 10 wasmeasured under the above conditions. As a result, the reflectance was38.2%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.66, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that excellent recording/reading characteristics canbe obtained even when the material of the buffer layer is changed froman Ag alloy to ZnS—SiO₂.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whenthe groove recording is performed like Example 10. To the contrary, itwas found that the characteristics necessary for recording/reading canbe obtained when the land recording is performed like this Example 16.

Example 17

In Example 17, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 11 wasmeasured under the above conditions. As a result, the reflectance was36.1%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.73, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that excellent recording/reading characteristics canbe obtained even when the material of the buffer layer is changed froman Ag alloy to SiO₂.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whenthe groove recording is performed like Example 11. To the contrary, itwas found that the characteristics necessary for recording/reading canbe obtained when the land recording is performed like this Example 17.

Example 18

In Example 18, on the second substrate, a guide groove was formed tohave a groove depth of 30 nm (corresponding to approximately λ/20), agroove width (G width) of 220 nm and a land width (L width) of 520 nm.

An Ag alloy was spattered to form the buffer layer. A metal complex azodye having a dye concentration of 3.10 wt % as being a coating conditionwas spin-coated to form the second recording layer.

The film thickness (thick film portion, G film thickness) of the grooveportion of the second recording layer formed as this was 110 nm, and thefilm thickness (thin film portion, L film thickness) of the land portionof the same was 70 nm.

The reflectance at the groove portion of the optical recording mediumprepared as above was measured under the above conditions. Thereflectance was 43.0%, as shown in Table 1.

It was found that the reflectance increases as the groove depth on thesecond substrate decreases, as compared with the above Example 7.

Example 19

In Example 19, the reflectance was measured in the same manner as theabove Example 18 excepting that the material of the buffer layer wasZnS—SiO₂. The reflectance was 39.0%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.22, as shown in Table 1.However, since the maximum signal amplitude can be improved by adjustingthe film thickness and the like of the recording layer, it is consideredthat an optical recording medium like this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to ZnS—SiO₂.

Example 20

In Example 20, the reflectance was measured in the same manner as theabove Example 18 excepting that the material of the buffer layer wasSiO₂. The reflectance was 43.2%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.11, as shown in Table 1.However, since the maximum signal amplitude can be improved by adjustingthe film thickness and the like of the recording layer, it is consideredthat an optical recording medium like this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to SiO₂.

Example 21

In Example 21, the reflectance was measured in the same manner as theabove Example 18 excepting that the dye concentration as being thecoating condition was 3.55 wt %, whereby the film thickness (thick filmportion, G film thickness) of the groove portion of the second recordinglayer was 135 nm, and the film thickness (thin film portion, L filmthickness) of the land portion of the same was 90 nm (that is, adifference in film thickness between the thick film portion and the thinfilm portion of the second recording layer was 45 nm). The reflectancewas 30.2%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was a little smaller than 0.1 (whenperforming the recording at a recording power of 15 mW). However, sincethe maximum signal amplitude can be improved by adjusting the filmthickness and the like of the recording layer, it is considered that anoptical recording medium like this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the film thickness of the second recording layer isvaried.

Example 22

In Example 22, the reflectance was measured in the same manner as theabove Example 21 excepting that the material of the buffer layer wasZnS—SiO₂. The reflectance was 41.7%, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to ZnS—SiO₂.

Example 23

In Example 23, the reflectance was measured in the same manner as theabove Example 20 excepting that the dye concentration as being thecoating condition was 3.55 wt %, whereby the film thickness (thick filmportion, G film thickness) of the groove portion of the second recordinglayer was 135 nm, and the film thickness (thin film portion, L filmportion) of the land portion of the same was 90 nm (that is, adifference in film thickness between the thick film portion and the thinfilm portion was 45 nm). The reflectance was 38.9%, as shown in Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.17, as shown in Table 1.However, since the maximum signal amplitude can be improved by adjustingthe film thickness and the like of the recording layer, it is consideredthat an optical recording medium like this is usable.

It was found that reflectance necessary for recording/reading can beobtained even when the film thickness of the second recording layer isvaried.

Example 24

In Example 24, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 18 wasmeasured under the above conditions. The reflectance was 49.1%, as shownin Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

Example 25

In Example 25, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 19 wasmeasured under the above conditions. The reflectance was 43.9%, as shownin Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.72, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the material of the buffer layer is changed from anAg alloy to ZnS—SiO₂.

It was also found that more excellent recording/reading characteristicscan be obtained when the film thickness, the groove width and the landwidth of the second recording layer are changed in the case of aZnS—SiO₂ buffer layer.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whenthe groove recording is performed like Example 19. To the contrary, itwas found that the characteristics necessary for recording/reading canbe obtained when the land recording is performed like this Example 25.

Example 26

In Example 26, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 20 wasmeasured under the above conditions. The reflectance was 49.5%, as shownin Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.58, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that when the material of the buffer layer is changedfrom ZnS—SiO₂ to SiO₂, the maximum signal amplitude is slightlydeteriorated although the reflectance is improved. However, it was foundthat the characteristics necessary for recording/reading can beobtained.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whenthe groove recording is performed like Example 20. To the contrary, itwas found that the characteristics necessary for recording/reading canbe obtained when the land recording is performed like this Example 26.

Example 27

In Example 27, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 21 wasmeasured under the above conditions. The reflectance was 33.9%, as shownin Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.79, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that when the material of the buffer layer is changedfrom ZnS—SiO₂ to an Ag alloy and the film thickness of the secondrecording layer is varied, the maximum signal amplitude is improved andthe characteristics necessary for recording/reading can be obtainedalthough the reflectance is slightly deteriorated.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be obtained whenthe groove recording is performed like Example 21. To the contrary, itwas found that the characteristics necessary for recording/reading canbe obtained when the land recording is performed like this Example 27.

Example 28

In Example 28, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 22 wasmeasured under the above conditions. The reflectance was 47.9%, as shownin Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

Example 29

In Example 29, the reflectance at the land portion of an opticalrecording medium prepared in the same manner as the above Example 23 wasmeasured under the above conditions. The reflectance was 44.2%, as shownin Table 1.

The polarity and the maximum signal amplitude of recorded signals weremeasured. The polarity of the recorded signals was H to L, and themaximum signal amplitude of the same was 0.55, as shown in Table 1.

It was found that reflectance necessary for recording/reading can beobtained even when the groove recording is changed to the landrecording.

It was also found that when the material of the buffer layer is changedfrom an Ag alloy to SiO₂, the reflectance is improved and thecharacteristics necessary for recording/reading can be obtained althoughthe maximum signal amplitude is slightly deteriorated.

Even in an optical recording medium prepared in the same manner,characteristics necessary for recording/reading cannot be contained whenthe groove recording is performed like Example 23. To the contrary, itwas found that when the land recording is performed like this Example29, the characteristics necessary for recording/reading can be obtained.

Comparative Example 1

In Comparative Example 1, on the second substrate, a guide groove wasformed to have a groove depth of 120 nm (corresponding to approximatelyλ/5.5), a groove width (G width) of 330 nm and a land width (L width) of410 nm.

An Ag alloy was spattered to form the buffer layer. A metal complex azodye having a concentration of 1.90 wt % as the coating condition wasspin-coated to form the second recording layer.

The film thickness (thick film portion, G film thickness) of the grooveportion of the second recording layer formed as this was 70 nm, and thefilm thickness (thin film portion, L film thickness) of the land portionof the same was 30 nm.

The reflectance at the groove portion of an optical recording mediumprepared as above was measured under the above conditions. Thereflectance was 9.0%, as shown in Table 1.

It was found that when the depth of the groove on the second substrateis large, reflectance necessary for recording/reading cannot beobtained, as compared with the above examples.

Comparative Example 2

In Comparative Example 2, the reflectance was measured in the samemanner as the above Comparative Example 1 excepting that the material ofthe buffer layer was SiO₂. The reflectance was 6.5%, as shown in Table1.

It was found that reflectance necessary for recording/reading cannot beobtained even when the material of the buffer layer is changed from anAg alloy to SiO₂.

Comparative Example 3

In Comparative Example 3, on the second substrate, a guide groove wasformed to have a groove width of 160 nm (corresponding to approximatelyλ/4), a groove width (G width) of 310 nm and a land width (L width) of430 nm.

An Ag alloy was spattered to form the buffer layer. A metal complex azodye having a dye concentration of 1.90 wt % as the coating condition wasspin-coated to form the second recording layer.

The film thickness (thick film portion, G film thickness) of the grooveportion of the second recording layer formed as above was 75 nm, and thefilm thickness (thin film portion, L film thickness) of the land portionof the same was 20 nm.

The reflectance at the groove portion of an optical recording mediumprepared as above was measured under the above conditions. Thereflectance was 12.9%, as shown in Table 1.

It was found that when the depth of the groove on the second substrateis increased, reflectance necessary for recording/reading cannot beobtained although the reflectance increases, as compared with the aboveComparative Example 1.

Comparative Example 4

In Comparative Example 4, the reflectance was measured in the samemanner as the above Comparative Example 3 excepting that the material ofthe buffer layer was SiO₂. The reflectance was 19.9%, as shown in Table1.

It was found that even when the material of the buffer layer is changedfrom an Ag alloy to SiO₂, reflectance necessary for recording/readingcannot be obtained although the reflectance increases.

CONCLUSION

It was found that when the depth of the groove on the second substrateis, for example, 120 nm or 160 nm like the above Comparative Examples 1through 4, reflectance necessary to record or read information in orfrom the second dye containing recording layer positioning farther froma side from which the light beam comes in cannot be obtained. To thecontrary, it was found that when the depth of the groove on the secondsubstrate is decreased to, for example, not greater than 65 nm like theabove Examples 1 through 29, reflectance necessary to record or readinformation in or from the second dye containing recording layerpositioning farther from the side from which the light beam comes in canbe obtaining in both the land recording and the groove recording.

It was found that, in the above Examples 1, 8 through 11, 19 through 21and 23, recording/reading characteristics (polarity, maximum signalamplitude) necessary to record or read information in or from the seconddye containing recording layer positioning farther from the side fromwhich the light beam comes in are difficult to be obtained in the grooverecording. To the contrary, it was found that, in the above Examples 2,14 through 17, 25 through 27 and 29, the recording/readingcharacteristics (polarity, maximum signal amplitude) necessary to recordor read information in or from the second dye containing recording layerpositioning farther from the side from which the light beam comes in canbe obtained in the land recording.

In each of the above examples, a groove-less substrate without therecording layer and the semitransparent reflective layer was used as thefirst information recording body in order to eliminate an effect of thefirst information recording body and evaluate the characteristics of thesecond recording layer as accurately as possible. However, use of ageneral first information recording body does not largely exert aneffect on the evaluation on the second recording layer.

In the media used in Examples 1 through 29, sufficient reflectance,which is the most important requirement for recording/reading, can beobtained. It is considered that an optical recording media having otherexcellent recording/reading characteristics can be obtained by suitablyselecting a structure other than the groove depth.

In Examples 2, 14 through 17, 25 through 27 and 29, it is consideredthat optical recording media having other excellent recording/readingcharacteristics can be obtained by suitably selecting a structure otherthan the recording layer film thickness.

This application is based on Japanese Patent Application Number2003-109486 filed on Apr. 14, 2003, and Japanese Patent Application No.2003-110579 filed on Apr. 15, 2003, the whole contents of which arehereby incorporated by reference.

1. A recording/reading method for an optical recording medium comprising a first dye containing recording layer and a second dye containing recording layer, in which information is recorded in or read from said first dye containing recording layer and said second dye containing recording layer, which has a thick film portion and a thin film portion, by irradiating a light beam from one side thereof, said recording/reading method comprising the steps of: irradiating said light beam to said thin film portion of said second dye containing recording layer through said first dye containing recording layer to record or read information in or from said second dye containing recording layer.
 2. The recording/reading method for an optical recording medium according to claim 1, wherein said thick film portion and said thin film portion of said second dye containing recording layer are formed correspondingly to a concave portion and a convex portion, respectively, on a substrate formed on an opposite side to said side, from which said light beam is irradiated.
 3. The recording/reading method for an optical recording medium according to claim 1 comprising the steps of: irradiating said light beam to a thick film portion of said first dye containing recording layer to record or read information in or from said first dye containing recording layer.
 4. The recording/reading method for an optical recording medium according to claim 3, wherein said thick film portion and said thin film portion of said first dye containing recording layer are formed correspondingly to a concave portion and a convex portion, respectively, on a substrate formed on said side, from which said light beam is irradiated. 